Fold libgjs-gdbus.so into libgjs.so

There's no strong reason to have it be a separate shared library,
and having private shared libraries in the main $(libdir) is
frowned upon.

We should also install Gjs-DBus.typelib into a package-private
typelib directory.  Fortunately, gjs_context_constructor() already
had the code to push PKGLIBDIR into the girepository search path.

https://bugzilla.gnome.org/show_bug.cgi?id=674756