Rework dependency registration

Change the way the dependencies are registred so that now it follows the same
mechanism as when the *_DEPENDS making the dependency registration system ready
to allow multiple packages from the same origin such as flavours and subpackages

Also allowing to install twice the same port one built with python33 and the
other built with python27 for example.

PR: 19702
With help of: antoine
Exp-run: portmgr (antoine)