Redefine G_PART_SCHEME_DECLARE() from populating a private linker set

Redefine G_PART_SCHEME_DECLARE() from populating a private linker set
to declaring a proper module. The module event handler is part of the
gpart core and will add the scheme to an internal list on module load
and will remove the scheme from the internal list on module unload.
This makes it possible to dynamically load and unload partitioning
schemes.