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).
fd02a140c4