fix: dependency resolver on windows only works when --enable_runfiles and --windows_enable_symlinks is used

[ChewyGumball]

compile_pip_requirements doesn't work as expected on Windows unless both --enable_runfiles and --windows_enable_symlinks are used. Both options default to off on Windows because the filesystem on Windows makes setting up the runfiles directories with actual files very slow. This means that anyone on Windows with a default set up has to search around the Github issues to try and figure out why things don't work as advertised.

The dependency_resolver.py doesn't inherently require these options, it just had two bugs that prevented it from working.

1. calling pip_compile exits the whole program so it never gets to run the code that should copy the output to the source tree. Things just happen to work on linux because the runfiles are symlinks, and it does not need to copy anything.
2. it assumed the runfiles resolved file would be in the runfiles tree, but on Windows, when --enable_runfiles is not set, it actually gets resolved to a file in the source tree.

Before:

bazel run //third_party/python:requirements.update
Starting local Bazel server and connecting to it...
INFO: Invocation ID: 8aa3e832-78ce-4999-912b-c43e7ca3212b
INFO: Analyzed target //third_party/python:requirements.update (129 packages loaded, 9563 targets configured).
INFO: Found 1 target...
Target //third_party/python:requirements.update up-to-date:
  bazel-bin/third_party/python/requirements.update.zip
  bazel-bin/third_party/python/requirements.update.exe
INFO: Elapsed time: 60.964s, Critical Path: 0.77s
INFO: 8 processes: 2 remote cache hit, 6 internal.
INFO: Build completed successfully, 8 total actions
INFO: Running command line: bazel-bin/third_party/python/requirements.update.exe '--src=_main/third_party/python/requirements.txt' _main/third_party/python/requirements_lock.txt //third_party/python:requirements.update '--resolver=backtracking' --allow-unsafe --generate-hashes '--requirements-windows=_main/third_party/python/requirements_windows.txt' --strip-extras
Updating third_party/python/requirements_windows.txt
Error: Could not open file 'third_party/python/requirements_windows.txt': No such file or directory

After:

bazel run //third_party/python:requirements.update
INFO: Invocation ID: 39f999a0-6c1d-4b2c-a1be-3d71e838916a
INFO: Analyzed target //third_party/python:requirements.update (5 packages loaded, 45 targets configured).
INFO: Found 1 target...
Target //third_party/python:requirements.update up-to-date:
  bazel-bin/third_party/python/requirements.update.zip
  bazel-bin/third_party/python/requirements.update.exe
INFO: Elapsed time: 5.410s, Critical Path: 4.79s
INFO: 2 processes: 1 internal, 1 local.
INFO: Build completed successfully, 2 total actions
INFO: Running command line: bazel-bin/third_party/python/requirements.update.exe '--src=_main/third_party/python/requirements.txt' _main/third_party/python/requirements_lock.txt //third_party/python:requirements.update '--resolver=backtracking' --allow-unsafe --generate-hashes '--requirements-windows=_main/third_party/python/requirements_windows.txt' --strip-extras
Updating third_party/python/requirements_windows.txt
#
# This file is autogenerated by pip-compile with Python 3.13
# by the following command:
#
#    bazel run //third_party/python:requirements.update
#
mpmath==1.3.0 \
    --hash=sha256:7a28eb2a9774d00c7bc92411c19a89209d5da7c4c9a9e227be8330a23a25b91f \
    --hash=sha256:a0b2b9fe80bbcd81a6647ff13108738cfb482d481d826cc0e02f5b35e5c88d2c
    # via sympy
sympy==1.13.3 \
    --hash=sha256:54612cf55a62755ee71824ce692986f23c88ffa77207b30c1368eda4a7060f73 \
    --hash=sha256:b27fd2c6530e0ab39e275fc9b683895367e51d5da91baa8d3d64db2565fec4d9
    # via -r G:/projects/bedrock-engine/third_party/python/requirements.txt

And //third_part/python:requirements_windows.txt is updated.

Reference issues: #1943 #1431

[aignas]

Thanks for the PR.

• Could you please add documentation as part of this PR to the https://github.com/bazelbuild/rules_python/blob/main/docs/pypi-dependencies.md file to explicitly state what the minimum requirements for Windows are? Do we still require these options?
• Maybe it would be a good idea to change the .bazelrc in the examples to drop the options to ensure that we are testing the Windows part in the CI? What do you think?

[ChewyGumball]

* Could you please add documentation as part of this PR to the https://github.com/bazelbuild/rules_python/blob/main/docs/pypi-dependencies.md file to explicitly state what the minimum requirements for Windows are? Do we still require these options?

There are no longer any minimum requirements on Windows after this PR. I don't see any such existing requirements in that document, so I don't think we need to add anything do we?

* Maybe it would be a good idea to change the `.bazelrc` in the examples to drop the options to ensure that we are testing the Windows part in the CI? What do you think?

The problem is that the test created by compile_pip_requirements doesn't update the file in the source tree, so the existing integration tests can't cover this issue. I don't know how to make a test that would cover this issue because we would need to run the compile_pip_requirements.update target, and I don't think you can run a target from a test.

[aignas]

I'll play with the CI a little later, for now I wanted to leave the comment on the documentation.