Suppress unknown-name for global declarations the function itself defines

[nitishagar]

Fixes #4785

Problem

A global declaration inside a function errored with unknown-name ("Could not find name x") whenever the module had no module-level definition of x — even when the declaring function assigns x itself:

def set_workload_id(value: str) -> None:
    global workload_id  # ERROR: Could not find name `workload_id`
    workload_id = value

global x; x = value is a legal way to create the module-level name at runtime, so the declaration should not error.

Why it happened

The mutable-capture lookup for global names consults module-scope statics (plus a builtin fallback) only, and a function's global-assign intentionally never publishes a module-scope static — so the lookup cannot find the very name the function is about to define.

Change

The definitions phase already merges every definition site in a scope, so it knows whether the scope gives the name a value beyond the capture declaration:

• Definition records whether any definition site is a value definition (assignment or import; del and other capture declarations do not count).
• The flag is threaded through the MutableCapture static.
• declare_mutable_capture suppresses only the not-found error for global declarations in function/method scopes whose own body defines the name.

Everything else is unchanged: undefined reads (global a; print(a) with no definition anywhere), assignments textually before the declaration (x = 1; global x), module top-level and class-body declarations, and all nonlocal behavior keep erroring exactly as before.

Test plan

• New testcase!s in pyrefly/lib/test/scope.rs: the issue repro, undefined-read still errors, nested-if assignments (both orders), del doesn't define, assignment-before-declaration keeps its error, module top-level and class-body keep erroring
• cargo test -p pyrefly --lib green (the two pre-existing missing-source LSP interaction failures also fail on unmodified main)
• Formatting/linting clean

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D118601767. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

============================================================
SUMMARY
============================================================
Total: +0 new errors, -41 fixed errors
By preset: +0/-41 (default), +0/-41 (strict)

Projects with changes (9):
  mitmproxy: +0 -1
  spark: +0 -2
  pycryptodome: +0 -12
  setuptools: +0 -2
  asynq: +0 -1
  stone: +0 -3
  dd-trace-py: +0 -2
  pandas: +0 -17
  pyodide: +0 -1
============================================================

FULL DIFF DETAILS
------------------------------------------------------------

mitmproxy (https://github.com/mitmproxy/mitmproxy)
- ERROR test/helper_tools/memoryleak.py:32:18-21: Could not find name `ssl` [unknown-name]

spark (https://github.com/apache/spark)
- ERROR python/pyspark/serializers.py:369:16-31: Could not find name `_old_namedtuple` [unknown-name]
- ERROR python/pyspark/serializers.py:370:16-42: Could not find name `_old_namedtuple_kwdefaults` [unknown-name]

pycryptodome (https://github.com/Legrandin/pycryptodome)
- ERROR lib/Crypto/SelfTest/Cipher/test_pkcs1_oaep.py:329:20-25: Could not find name `asked` [unknown-name]
- ERROR lib/Crypto/SelfTest/Cipher/test_pkcs1_oaep.py:332:24-29: Could not find name `asked` [unknown-name]
- ERROR lib/Crypto/SelfTest/Cipher/test_pkcs1_oaep.py:357:20-28: Could not find name `mgfcalls` [unknown-name]
- ERROR lib/Crypto/SelfTest/Cipher/test_pkcs1_oaep.py:361:24-32: Could not find name `mgfcalls` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_DSA.py:71:16-19: Could not find name `DSA` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_DSA.py:71:21-27: Could not find name `Random` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_DSA.py:71:29-42: Could not find name `bytes_to_long` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_DSA.py:71:44-48: Could not find name `size` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_RSA.py:90:16-19: Could not find name `RSA` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_RSA.py:90:21-27: Could not find name `Random` [unknown-name]
- ERROR lib/Crypto/SelfTest/PublicKey/test_RSA.py:90:29-42: Could not find name `bytes_to_long` [unknown-name]
- ERROR lib/Crypto/SelfTest/Util/test_Counter.py:33:16-23: Could not find name `Counter` [unknown-name]

setuptools (https://github.com/pypa/setuptools)
- ERROR setuptools/tests/test_setuptools.py:54:20-21: Could not find name `x` [unknown-name]
- ERROR setuptools/tests/test_setuptools.py:54:23-24: Could not find name `y` [unknown-name]

asynq (https://github.com/quora/asynq)
- ERROR asynq/tests/test_contexts.py:104:12-39: Could not find name `expected_change_amount_base` [unknown-name]

stone (https://github.com/dropbox/stone)
- ERROR stone/_vendor/ply/lex.py:868:12-17: Could not find name `lexer` [unknown-name]
- ERROR stone/_vendor/ply/lex.py:874:12-17: Could not find name `token` [unknown-name]
- ERROR stone/_vendor/ply/yacc.py:3168:12-17: Could not find name `parse` [unknown-name]

dd-trace-py (https://github.com/DataDog/dd-trace-py)
- ERROR ddtrace/internal/process_tags/__init__.py:140:12-24: Could not find name `process_tags` [unknown-name]
- ERROR ddtrace/internal/process_tags/__init__.py:141:12-29: Could not find name `process_tags_list` [unknown-name]

pandas (https://github.com/pandas-dev/pandas)
- ERROR pandas/io/clipboard/__init__.py:140:12-24: Could not find name `QApplication` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:348:12-19: Could not find name `HGLOBAL` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:348:21-27: Could not find name `LPVOID` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:348:29-34: Could not find name `DWORD` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:348:36-42: Could not find name `LPCSTR` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:348:44-47: Could not find name `INT` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:349:12-16: Could not find name `HWND` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:349:18-27: Could not find name `HINSTANCE` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:349:29-34: Could not find name `HMENU` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:349:36-40: Could not find name `BOOL` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:349:42-46: Could not find name `UINT` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:349:48-54: Could not find name `HANDLE` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:540:12-22: Could not find name `Foundation` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:540:24-30: Could not find name `AppKit` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:540:32-36: Could not find name `qtpy` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:540:38-43: Could not find name `PyQt4` [unknown-name]
- ERROR pandas/io/clipboard/__init__.py:540:45-50: Could not find name `PyQt5` [unknown-name]

pyodide (https://github.com/pyodide/pyodide)
- ERROR src/tests/test_jsproxy.py:2956:12-13: Could not find name `A` [unknown-name]