feat: add `git_fetch` repository rule

This patch also adds:

• git_dir: create a .git declared directory from a filegroup
• git_commit: represent a git commit as a Bazel provider
• git_checkout: checkout a commit into a declared directory

These rules are stamped out in the git_fetch repository.

The network access is performed in the repository context to retrieve the .git objects. The rest of the git operations are performed in the action context. This has many benefits:

• On-demand I/O when actions are requested
• Separation of concerns for different git operations
• Remote cache and remote execution friendly actions

The rules use rules_coreutils and bazel/git> to be hermetic.