replay: Use remote repository base URL to export external located in it

Some externals might be located in the same repository we are
currently loading. In that case, replace the external base URL
which corresponds to the origin URL by the remote repository URL.

When we use SvnLoaderFromRemoteDump, that remote repository URL
corresponds to a local repository mounted from a dump file so we
can export the external path in a much faster way without any costly
network requests.

Related to T611