provide a hook inside fetch_remote_file for ease mass file fetching

I'd like to see a hook inside `fetch_remote_file`.
That way I could substitute things like `https://domain.net/files/FOO` by `file:///locally-synchronized/FOO`
and dramatically increase importation (especially testing importations).
Playing with `$remote_url` is not an option since it's used later for remapping.

Start by locating fetch_remote_file and reading how $remote_url is used before and after the fetch. Trace its callers and existing tests, if present. Done means a hook can substitute local file URLs for remote URLs while preserving the later remapping behavior.

Written by the indexing model from the issue text.