Make tools/make_version.py hermetic and eliminate two-list input drift

[kevmoo]

tools/make_version.py resolves several of its inputs implicitly:

• tools/utils.py:92–93 computes DART_DIR = abspath(__file__ + '/../..') and
  VERSION_FILE = os.path.join(DART_DIR, 'tools', 'VERSION').
• make_version.py reads runtime/vm/<file> for each entry in
  VM_SNAPSHOT_FILES (a hardcoded list in the script) to compute the snapshot
  MD5 hash.
• The script shells out to git from inside DART_DIR for the SDK hash
  unless --no-git-hash is passed.

Consequences:

• The script can only run from a checked-out SDK tree at the expected on-disk
  layout. Vendored Dart copies, sandboxed CI builds, and any build system that
  stages inputs into a different directory all need workarounds.
• The script's input set is not declared anywhere a build system can read.
  GN today approximates this by listing the snapshot files manually in
  runtime/BUILD.gn:419–435's inputs = [...] block. The two lists (the
  Python VM_SNAPSHOT_FILES constant and the GN inputs array) must stay in
  sync by convention, with no enforcement. They have drifted before and will
  drift again.
• CI builds that want a deterministic version string must work around the
  implicit git shell-out (e.g., by clobbering PATH).

Why this is an improvement on its own

• Declarative inputs make incremental rebuilds correct: editing dart.cc
  triggers a version.cc regeneration regardless of build system.
• The script becomes reusable: vendored Dart, custom embedders, code-search
  indexers, internal CI pipelines all benefit.
• Removes the implicit two-list drift between VM_SNAPSHOT_FILES in Python and
  inputs = [...] in GN.
• A single source of truth for which files affect snapshot compatibility.

How it makes Bazel (and any other non-GN build) easier

Bazel genrule()s require every input to be declared in srcs so the
sandbox can stage it. Matching the staged file set against the Python
VM_SNAPSHOT_FILES list is fragile — any new entry on either side breaks the
genrule silently or with a confusing error. With explicit CLI flags, the
genrule passes its declared inputs directly.

Proposed change (sketch)

Add CLI flags to make_version.py and utils.py:

• --dart-dir <path> — overrides __file__-derived DART_DIR
• --version-file <path> — overrides tools/VERSION lookup (already partially
  threaded through --version-file; complete the plumbing)
• --snapshot-files <list> — explicit set of files to hash for
  {{SNAPSHOT_HASH}}, replacing the hardcoded VM_SNAPSHOT_FILES constant
• --git-hash <hash-or-empty> — accept the SDK hash explicitly instead of
  shelling out to git

Keep current behavior as defaults so existing callers don't break.

Affected code

• tools/make_version.py — input resolution, arg parsing, VM_SNAPSHOT_FILES
  constant
• tools/utils.py:92–93 — DART_DIR, VERSION_FILE resolution
• tools/utils.py:362,380,419,439,454,469 — GetVersion, GetChannel,
  GetGitRevision, GetShortGitHash, GetGitTimestamp,
  GetGitTimestampForDpkg — all anchor on DART_DIR
• runtime/BUILD.gn:410–455 — action("generate_version_cc_file"), the
  inputs = [...] list that has to be kept in sync with VM_SNAPSHOT_FILES