Add helper functions for module extensions as `modules`

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Adds a new module modules with two helper functions for module extensions:

• use_all_repos makes it easy to return an appropriate extension_metadata from a module extension (if supported) to indicate that all repositories generated by the extension should be imported via use_repo.
• as_extension turns a WORKSPACE macro into a module extension that uses use_all_repos to automate the generation of use_repo calls.

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@Wyverald Could you review this from the Bzlmod dev perspective? @tetromino Could you review this from the Bzlmod user perspective?

I can also add full e2e tests if desired.

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@Wyverald Friendly ping

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CI is failing due to the missing Python toolchain registration.

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@Wyverald Tests are passing now.

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@Wyverald Friendly ping

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After the release of 7.1.0, I will add reproducible.