Prevent the resource manager from installing a resource twice

Well, specifically, this prevents the resource manager from installing a
resource which has the same name as a resource existing already on the
related resource server.

This can happen, amongst others, if you modify a bundle resource and
save it. Or if you install a bundle with resources that you already had
installed. This prevents the resource manager from overwriting both
cases.
If necessary we'll think up something different, but this works for now.

tl;dr: No duplicated resources.

CCBUG:341752