Starlarkify all repository rules

[Wyverald]

We originally wanted to allow native repo rule usage in module extensions (#15412), but that turned out to be surprisingly hard to implement.

Instead, we could just go down the other route: Starlarkify all the repo rules!

• local_repository (trivial) (added in 6f254ce)
• local_config_platform (trivial; but also kind of unnecessary, since nobody should use it other than @local_config_platform itself) (done in Implement local_config_platform in @platforms platforms#86)
• Add some sort of repository_ctx API to acquire a DirectoryListingValue (added in 7.1.0: path.readdir) This would unblock the following:
  • new_local_repository (added in 6f254ce)
  • android_sdk_repository (now in https://github.com/bazelbuild/rules_android)
  • android_ndk_repository (partially done: https://github.com/bazelbuild/rules_android_ndk)

[Wyverald]

cc @ted-xie @ahumesky @kotlaja

[fmeum]

@Wyverald Starlarkifying local_config_platform would get us halfway to #8766 (comment), so I think it's worth doing.

[ahumesky]

A starkark version of android_ndk_repository is mostly complete here: https://github.com/bazelbuild/rules_android_ndk

[Wyverald]

See https://docs.google.com/document/d/17RZKMuMjAIgNfFdrsLhqNsMTQDSS-BBA-S-fCSBPV7M/edit# for the design to address the missing API.