Fix fetching non-expanded resources from mirrors (#1310)

This closes #1308, where fetching a non-expanded resource from a mirror
will cause an error.

This also ensures that when a URL resource is fetched from a mirror,
that it will be named as though it were retrieved from the original
URL. This is particularly useful for non-expanded resources since it
ensures that the resource name is consistent for the installation
(this is less important for expanded resources because the build takes
place inside the expanded resource).