(PUP-2039) Fix rpm package provider usage of --whatprovides.

The fix for PUP-897 (commit ecb761e24a) introduced always passing
--whatprovides to rpm when querying for packages. This breaks querying
for packages that have architectures specified, e.g.
'vim-enhanced.x86_64'.

The proper way to do this is to first query without --whatprovides and
see if rpm finds a package by the given name.  If rpm fails to find a
package, fallback to using --whatprovides to support querying for
virtual package names.
Merge pull request #2476 from peterhuene/pup/2039

(PUP-2039) Fix rpm package provider usage of --whatprovides.