feat(whl_install): expose install_dir via OutputGroupInfo (#926)

## Problem

After #907 removed `install_dir` from `DefaultInfo.files`, consumers
that need to access files within the wheel directory directly (e.g.
extracting a non-console-script binary like `py-spy`) have no way to
reference the install directory from a genrule `srcs`.

## Fix

Add `OutputGroupInfo(install_dir = depset([install_dir]))` to the
`whl_install` rule's return value. Consumers can access the directory
via a `filegroup` with `output_group = "install_dir"`:

```python
filegroup(
    name = "py_spy_install_dir",
    srcs = ["@pip//py_spy:install"],
    output_group = "install_dir",
)

genrule(
    name = "py_spy_cli",
    srcs = [":py_spy_install_dir"],
    outs = ["py-spy"],
    cmd = "cp $(execpath :py_spy_install_dir)/bin/py-spy $@",
)
```

This keeps `DefaultInfo.files` empty (preserving the improvement from
#907) while still making the install directory reachable for the rare
cases that need it.

## Test plan

- [x] Existing e2e suite passes
- [x] New e2e test case `uv-whl-install-output-group`: installs
`iniconfig` via uv, accesses the install dir through
`filegroup(output_group = "install_dir")`, uses it as a `genrule` srcs,
and asserts the listing contains expected package files

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gregmagolan

e2e/MODULE.bazel

@@ -249,6 +249,18 @@ uv.project(
 )
 # }}}
 
+# For cases/uv-whl-install-output-group
+# Verify that whl_install exposes install_dir via OutputGroupInfo so that a
+# filegroup with output_group = "install_dir" makes the wheel directory
+# accessible to downstream genrules.
+# {{{
+uv.project(
+    hub_name = "pypi",
+    lock = "//cases/uv-whl-install-output-group:uv.lock",
+    pyproject = "//cases/uv-whl-install-output-group:pyproject.toml",
+)
+# }}}
+
 # For cases/uv-include-group (PEP 735 include-group syntax)
 # {{{
 uv.project(

e2e/cases/uv-whl-install-output-group/BUILD.bazel

@@ -0,0 +1,28 @@
+# Regression test: whl_install must expose install_dir via OutputGroupInfo.
+#
+# Since #907 removed install_dir from DefaultInfo.files, consumers that need to
+# access files from the wheel directory directly must use a filegroup with
+# output_group = "install_dir". This test exercises that pattern end-to-end.
+
+load("@aspect_rules_py//py/unstable:defs.bzl", "py_venv_test")
+
+# Access the install dir via the install_dir output group rather than
+# DefaultInfo.files (which is intentionally empty since #907).
+#
+# This filegroup is placed in py_venv_test's data (not a genrule srcs) so that
+# the venv transition applies and @pypi//iniconfig's hub select resolves.
+filegroup(
+    name = "iniconfig_install_dir",
+    srcs = ["@pypi//iniconfig"],
+    output_group = "install_dir",
+)
+
+py_venv_test(
+    name = "test",
+    srcs = ["test.py"],
+    data = [":iniconfig_install_dir"],
+    main = "test.py",
+    python_version = "3.11",
+    venv = "uv-whl-install-output-group",
+    deps = ["@pypi//iniconfig"],
+)

e2e/cases/uv-whl-install-output-group/pyproject.toml

@@ -0,0 +1,7 @@
+[project]
+name = "uv-whl-install-output-group"
+version = "0.0.0"
+requires-python = ">=3.11"
+dependencies = [
+    "iniconfig",
+]

e2e/cases/uv-whl-install-output-group/test.py

@@ -0,0 +1,35 @@
+"""Test that whl_install exposes install_dir via OutputGroupInfo.
+
+filegroup(output_group="install_dir") is placed in py_venv_test's data so the
+venv transition applies. If OutputGroupInfo.install_dir is not exposed, the
+filegroup produces no files and the TreeArtifact is absent from TEST_SRCDIR.
+"""
+
+import os
+import unittest
+
+
+class WhlInstallOutputGroupTest(unittest.TestCase):
+    def test_install_dir_accessible_via_output_group(self):
+        srcdir = os.environ["TEST_SRCDIR"]
+        # Find any .py file under the iniconfig install directory. The tree
+        # artifact is placed somewhere under TEST_SRCDIR by Bazel's runfiles.
+        found = []
+        for root, _dirs, files in os.walk(srcdir):
+            for f in files:
+                path = os.path.join(root, f)
+                if "iniconfig" in path and f.endswith(".py"):
+                    found.append(path)
+
+        self.assertGreater(
+            len(found),
+            0,
+            "No iniconfig .py files found under TEST_SRCDIR — "
+            "OutputGroupInfo.install_dir was not exposed or not propagated "
+            "through filegroup(output_group='install_dir')\n"
+            f"TEST_SRCDIR={srcdir}",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()

e2e/cases/uv-whl-install-output-group/uv.lock

@@ -74,6 +74,12 @@ def _whl_install(ctx):
                 install_dir,
             ]),
         ),
+        OutputGroupInfo(
+            # Expose install_dir for consumers that need to access files from
+            # the wheel directory directly (e.g. extracting non-console-script
+            # binaries via a filegroup with output_group = "install_dir").
+            install_dir = depset([install_dir]),
+        ),
         PyInfo(
             transitive_sources = depset([
                 install_dir,
@@ -133,6 +139,7 @@ lighter weight since the toolchain's files aren't inputs.
     ],
     provides = [
         DefaultInfo,
+        OutputGroupInfo,
         PyInfo,
     ],
 )