Implement a `git` repository time checkout rule

This project provides a git_checkout execution time rule using the fetched objects from git_fetch.

However, if a user is fetching a Bazel project they need it at repository time, not execution time for the BUILD.bazel to be parsed.

This is a ticket to implement a hermetic git repository rule that performs a repository time checkout of the working copy.

There is not a requirement to be compatible with the upstream git_repository and should follow the same API as download_utils where possible. To prevent confusion it may make sense to name it something different.

It should re-use the git_fetch repository rule to share the downloaded git objects.

A strawperson API:

git_fetch(
    name = "something",
    urls = ["..."],
)

# Creating a repository time working tree of the commit that was fetched
git_worktree(
    name = "checkout",
    objects = "@something",
)

Later, we could provide a git_repository that combines both rules and conforms to the upstream API.