Add gir targets to the dependencies sources attribute and other meson cleanups.

Adding the gir targs to the dependency object of the library allows us to be
able to use the dependency in a gnome.generate_gir for a library that consumes
our api regardless of if it's a system install or a subproject.

Testing Done:
Modified the gplugin subproject in pidgin and used these changes to verify that it fixed the current build issues over there.

Reviewed at https://reviews.imfreedom.org/r/939/