PEP 725: large update, address all feedback and open issues

This also references PEP 804, which address one of the most significant
open issues about external dependency names needing to be curated
and have canonical versions, rather than accepting everything that
parses as a valid package URL. Hence these two PEPs now cross-reference
each other. PEP 725 contains the packaging metadata, and PEP 804
the machinery and process around how that metadata is curated and
can be queried, mapped, etc.

Author: rgommers