Select statements

See the blueprint cl for more information.

Things still to do:
 - Support selecting on product variables and
   variants
 - Test/Support property struct reflection tags
   like arch_variant, path, and variant_prepend
 - Test that selects combine well with existing
   configurability mechanisms like arch:, target:,
   multilib:, python's version:, etc.

Bug: 323382414
Test: go tests
Change-Id: If5d1ea1ad0c4ebabffaea91d62e1a1c6f926a793