Do not use search using PackageKit by default

We shouldn't actually need to do this, as all packages need to be converted to
applications using the AppStream metadata. The only time this is useful is when
the search term matches the package description and not the application
description, keyword, or the comment in the desktop file.

In this instance, the application should probably be shipping an AppData file
and be refined by the AppStream metadata. Consider it another reason to ship an
AppData file :)

This typically speeds up searching by more that 600ms per request.
trivial: Add a sync version of gs_plugin_loader_search()
Add a test binary gnome-software-cmd which is useful for profiling

Using gnome-software-session was just too clunky for such low-level measurements.