Expose repository_cache to skylark so shared git repository cache can use it

#5928 tries to introduce a shared repository cache (between workspaces) for git_repository (like we have for download/download_and_extract).

The main gap there is where to write the shared cache.

The current aggreement with @aehlig is that we will write it under `$repository_cache/content_addressable/git/...`.
`$repository_cache` is the value that is specified by `--repository_cache` cmd-line flag (or its default value) and `git` is the key-type (joining the existing `sha1`/`sha256`).

None of the above is exposed to skylark. We have two options:
1. Expose `$repository_cache/content_addressable` to skylark
1.1. Pro- very generic and can be reused by more key types if they'll be added
1.2. Con- might cause issues if a repository rule will overwrite and maybe even poison an existing key-type (`sha256` for example)

2. Expose `$repository_cache/content_addressable/git` to skylark
2.1. Pro- very safe and doesn't require the rule to be aware of the environment
2.2. Con- maybe too specific

@aehlig is strongly leaning towards 2 but we thought it would be a good idea to loop in @laurentlb before we make a decision.
@laurentlb we'd appreciate hearing your thoughts since we really want to get this into 0.18.0

Start by reading issue #5928 and the handling of the --repository_cache command-line flag, then trace how repository rules are exposed to Skylark. The open decision is whether Skylark receives the content_addressable directory or the content_addressable/git directory; done requires an agreed exposure design for the shared git repository cache.

Written by the indexing model from the issue text.