Support git_repository sources in BCR

[armandomontanez]

Adds workarounds in the BCR validation script to properly support git_repository sources as a step forwards toward bazelbuild/bazel#23461.

[meteorcloudy]

@fmeum @Wyverald Any concerns on supporting git_repository in BCR?

[fmeum]

Not in general, but we have to be careful about what kind of refs we allow. Tags can be changed retroactively, so if we want the lockfiles to actually lock down the contents, we would need to restrict this to commits (living with the fact that the hash is just SHA-1).

[meteorcloudy]

That's a really good point! @armandomontanez Can you please add such a check? Basically, verify commit is specified and matches the commit hash pattern.

[armandomontanez]

Good point, added a handful of checks/constraints to enforce this.