The name the user gives to the asset fetcher should be the primary
controlling aspect of the cache location.  The possible locations
where the asset might be found are just an aiding component to the
asset retrieval.

This is, besides a fix, also an opmization, given that
_get_relative_dir() is now called only once, instead of once for each
possible location.
Signed-off-by: NCleber Rosa <crosa@redhat.com>