feat(pypi): enable pipstar by default

[aignas]

Before this PR we were using our python helpers to parse the whl METADATA for a
particular host (or experimentally - target) platform and the fixed dependency
list would be written to the BUILD.bazel files materialized by the
whl_library repository rule.

This PR adds extra plumbing to leverage the Starlark implementation of the
METADATA file parsing (a.k.a. pipstar) which moves the evaluation to analysis
phase as we will get the target platform details via the
env_marker_config_setting from the toolchain used for the dependencies.

Since users are normally working with a subset of platforms that the METADATA
would pull in, we are writing the list of packages from the requirements file to
a bzl file so that pipstar knows which packages to consider when parsing the
METADATA. This will ensure that bazel query continues to work.

Since just passing the list of packages to the whl_library would cause
refetches of all of the whl_library dependencies when we add or remove a
package, we pass a path where to load the symbol called packages from, which
means that only the analysis phase will be affected because the macros will be
re-evaluated if one adds or removes a package.

We still need to download the correct platform-specific wheels for the right
platform in order to fully resolve the referenced tickets. With this PR we are
deprecating the experimental_target_platforms attribute since it has no longer
any effect.

Fixes #2949
Work towards #260
Work towards #2241

[aignas]

Need to wait for the #3247 so that I can finish fixing the tests

[aignas]

/gemini review

[gemini-code-assist]

Code Review

This pull request enables pipstar by default for bzlmod builds, which is a significant step forward. The changes in internal_config_repo.bzl correctly implement this switch. The test updates, including modifications to hub_builder_tests.bzl and test data, align well with this new default, ensuring the test suite remains valid. The improved error logging in whl_metadata.bzl is also a nice addition for debuggability. I've found one potential issue regarding the use of dep_template in test dependencies which could cause problems in the future.

[aignas]

OK, PTAL.

[aignas]

/gemini review

[gemini-code-assist]

Code Review

This pull request enables the Starlark-based pipstar dependency resolver by default, a significant step towards moving dependency evaluation to the analysis phase. The changes include extensive plumbing to support this, such as introducing whl_config_repository to replace group_library, and passing package information via a config.bzl file. The experimental_target_platforms attribute is now deprecated. The implementation is robust, with considerations for backward compatibility, and the test suite has been thoroughly updated to reflect these substantial changes. My review found the changes to be well-executed, with only a minor suggestion to improve the clarity of the deprecation notice in the changelog.

[aignas]

Self review.

[aignas]

PTAL

[rickeylev]

Any known compatibility issues we should document, or should this be a transparent drop in replacement?

[aignas]

This should be a drop-in replacement, oh I think we need to point users to how they can configure the environment used for doing the evaluation.