Only stage required wheel packages from requirements cache, not entire cache directory

[NayanMathur411]

Fixes #35717

When using --requirements_file, the Python SDK downloads wheel packages to a cache directory (TEMPDIR/dataflow-requirements-cache), the stager would stage all packages in the cache directory, even those not needed by the current workflow. This caused unnecessary files to be staged and uploaded.

(cc @hjtran)

Changes

• Modified _populate_requirements_cache to download packages to a temporary directory first, then copy to cache. This allows tracking exactly which packages are needed for the current requirements file.
• Updated create_job_resources to only stage packages returned by _populate_requirements_cache, rather than globbing the entire cache directory.
• Renamed cache directory from dataflow-requirements-cache to beam-requirements-cache (runner-agnostic naming).
• Added test test_only_required_packages_staged_from_cache to verify the fix.

Example

Before this fix:

1. Run pipeline A with requirements.txt containing numpy, pandas → cache has numpy.whl, pandas.whl
2. Run pipeline B with requirements.txt containing scipy → cache now has numpy.whl, pandas.whl, scipy.whl
3. Pipeline B stages all 3 packages, even though only scipy is needed

After this fix:

• Workflow B only stages scipy.whl

Testing

• Existing stager_test.py tests pass
• New test test_only_required_packages_staged_from_cache verifies only required packages are staged

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