Move gobject sub-module Python files into the main gi package

This moves the signalhelper, propertyhelper, and constants Python modules
from gi/_gobject into gi. Keep gi/_gobject/__init__.py around because it is
still needed to maintain the "_PyGObject_API" exposed by pygobject.h. This
allows external modules compiled with prior versions of PyGObject to
continue working with newer versions.

https://bugzilla.gnome.org/show_bug.cgi?id=712197