Move libgirepository into GLib

The girepository API currently part of gobject-introspection doesn't really need to live in a separate library, as its functionality gives the ability to any GLib/GObject user to introspect the C ABI of a library. Non-C consumers of the binary introspection data—i.e. language bindings using the typelib format—don't really need to depend on the gobject-introspection project, as g-i also provides the C scanner and various other non-binding-related tools.

Ideally, gobject-introspection itself should move towards depending on the external libgirepository.

Helps: #3155 (closed)