giomodule: When loading GIO_EXTRA_MODULES skip duplicates

giomodule: When loading GIO_EXTRA_MODULES skip duplicates

 * Load modules from paths listed in GIO_EXTRA_MODULES environment
   variable first.
 * Ignore duplicate modules based on module basename.
 * Add the concept of GIOModuleScope which allows other callers to
   skip duplicate loaded modules, or block specific modules based on
   basename.
 * Document behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=656914