fix pex.pex, move away from ObtainerFactory and just use Obtainer

This keeps the Obtainer but instead moves away from using an ObtainerFactory
and pushes the smarts directly into application-specific Obtainers (e.g.
CachingObtainer, PantsObtainer, etc.)

This in turn fixes pex.pex (which has no tests!  mental note.)

Reviewed at https://rbcommons.com/s/twitter/r/154/