packagekit: stop supporting searches by package name

Back when we started supporting appstream a lot of poorly maintained
apps didn't appear in the databases, this made using Discover
unbearable, hence adding the feature of searching the package name from
the query.

Despite being against our vision, it made sense back then. Nowadays
applications should already have adopted AppStream in a way or another
and this shouldn't apply anymore.

BUG: 432690


(cherry picked from commit d1263d12)