CMSP 08. Extensibly Group Prereqs

[David Golden]

08. Extensibly Group Prereqs

Proposal:

Rather than have 'config_requires' and 'install_requires' and
'signature_requires' and 'build_recommends', have a two-level system.  This
requires a small bit of reworking now, but is easy to extend later without
adding many top-level keys.    Right now (phase)_(verb) is multiplicitive.
Both phases (test, install, configure) and verbs (requires, recommends,
conflicts) can be added easily over time.

Comments:

* I would also like to see this split better. It recently came up that a
  'recommends' list is inappropriate to list optional testing modules,
  however as a two-level system, 'test' and 'recommends' might be a better
  fit, as 'test_recommends' doesn't exist at the moment. (Barbie)

* This proposal assumes that the downstream consumers will be able to
  handle all possible combinations. The implications for the downstream are
  multiplicative, even if the encoding in META.yml are not.