parse name/version from pkg urls

[qwcode]

in many cases, we could parse the name & version from standard packages in pkg URL requirements,

i.e. know that peppercorn-1.4.tar.gz is project "peppercorn" and version "1.4"

and therefore have a real requirement object in the req attribute of our InstallRequirement objects, but we don't currently (and instead end up with an "unnamed" requirement in many cases), see #804 (comment) for context.

[chrahunt]

This was discussed as an optimization for #6607, so whether it's implemented in pip or we integrate with an implementation in packaging, it should probably be done as part of that effort.