Remove repository fork and import code from Gitlab::Shell

What does this MR do?

The RepositoryService.CreateRepositoryFromURL (aka import_repository) GRPC call is used when importing a project from a remote source. Historically, this was functionality we'd ask a subprocess in gitlab-shell to do. Now, Gitaly does it for us instead. The same is true for RepositoryService.ForkRepository.

Refactor the code so we no longer have to indirect via Gitlab::Shell to perform this call. It makes what is happening much clearer, and is a precursor to removing all references to Gitlab::Shell.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Part of #25095 (closed)