feat(pypi): add pip.dep to declare abstract pypi dependencies

[rickeylev]

Introduce the pip.dep tag class to allow modules to declare abstract
PyPI dependencies. These declarations ensure the target structure for
a PyPI package is created, but provide a no-op implementation that
passes analysis time, but fails at execution time.

Along the way, create an agent rule for *.bzl files to help guide it
in creating better bzl files.

[gemini-code-assist]

Code Review

This pull request introduces a new dep tag class to the pip bzlmod extension, enabling modules to declare abstract PyPI dependencies. This ensures that target structures exist in the unified hub without forcing a specific version or lock file on consumers, with downstream modules providing the concrete implementation via pip.parse. The feedback suggests a minor optimization in python/private/pypi/extension.bzl to remove a redundant normalize_name call since the keys of declared_deps are already normalized.

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[gemini-code-assist]

The keys of mods.declared_deps are already normalized when they are populated in parse_modules (using normalize_name(dep_attr.name)). Therefore, calling normalize_name(dep_name) here is redundant. We can simplify this loop by directly unpacking the normalized package name as norm_pkg.

Suggested change

	for dep_name, extra_targets in mods.declared_deps.items():
	norm_pkg = normalize_name(dep_name)
	for norm_pkg, extra_targets in mods.declared_deps.items():

[aignas]

Please add a unit test to ensure that if we declare a dep and then it is provided by a pip.parse, that it works as expected.