Epic: Implement "mixed" concept

[msarahan]

The "mixed" concept blends ideas from several posts on the python discourse forum. This concept is described in https://discuss.python.org/t/selecting-variant-wheels-according-to-a-semi-static-specification/53446/111

For a proof-of-concept implementation, there are several parts:

• Create some wheel files that contain proposed new metadata
• Repo (warehouse) changes to:
  • collect variant metadata when indexing a new package file
  • implement the 2 new views
• local hash generator:
  • come up with an example configuration file that covers the new metadata space in the example wheel files
  • Code the combinatoric expansion of variants and prioritization as listed in the discourse post
• Installer (pip):
  • Determine whether index priority is needed. If so, pursue index priority in a separate PEP.
  • Add iterative variant finding to pip's resolution process