remove overridesdir from the .pc file and add it to the gi module

* having the variable in the .pc file caused issues parallel installing
  for different versions of python
* putting it into the module allows us to give the correct directory
  based on which version of python you run the script from
* access the var as such:
    import gi
    installdir = gi._overridesdir

gi/__init__.py

@@ -25,8 +25,10 @@
 # Force loading the GObject typelib so we have available the wrappers for
 # base classes such as GInitiallyUnowned
 import gi._gobject
+import os
 
 _versions = {}
+_overridesdir = os.path.join(os.path.dirname(__file__), 'overrides')
 
 def require_version(namespace, version):
     repository = Repository.get_default()

pygobject-3.0.pc.in

@@ -9,11 +9,19 @@ libdir=@libdir@
 # pkg-config to get this value. You might want to use this to
 # install additional headers.
 pygobjectincludedir=${includedir}/pygobject-3.0
-overridesdir=@pyexecdir@/gi/overrides
 
 Name: PyGObject
 Description: Python bindings for GObject
 Requires: gobject-2.0
 Requires.private: @LIBFFI_PC@
 Version: @VERSION@
 Cflags: -I${pygobjectincludedir}
+
+# overridesdir has now moved to the gi module
+# third parties can access it in a python script:
+#
+# import gi
+# installdir = gi._overridesdir
+#
+# the version of python you run the script from
+# will determine the actual overrides path