Mark module extension as reproducible

[fmeum]

This removes an unnecessary lockfile entry.

The module extension is kept in place so that use_extension calls don't need to be updated. The repo rules are moved into a separate file so that WORKSPACE setups don't fail on a cyclic load of bazel_features. The newly exposed repo rules are protected by load visibility.

[cerisier]

Is this closer to getting merged ? I'm closing my quest of having the cleanest lockfile possible !

[keith]

can you rebase? I can merge after

[fmeum]

@keith done