software_manager: Search dict with lowercase distro name, add logging

The generic distribution package list is stored by with a lowercase
distro name key, using the detected_distro.name value unmodified on
Ubununtu results in failure as Ubuntu != ubuntu. Add some basic
logging to make this sort of failure much easier to identify in the
future.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
software manager: fallback to lowercase distro name

When looking for packages using distro names only (as opposed to
more detailed distro spec based lookup), fallback to distro names
expressed in lower case.

Also, only log that no generic package list was found if that's
actually the case.

Signed-off-by: Cleber Rosa <crosa@redhat.com>