Don't use Gitlab::Shell#import_repository on local paths

What does this MR do?

Changes Gitlab::Shell#import_repository to disallow local file imports.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This method currently either imports from a URL, or a local file. These are very different use cases from a Gitaly point of view. In the local file case we have to worry about how to copy the file to the Gitaly server.

So it makes our life easer if we don't mix these things up.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

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