Require %files section for package generation again

Require %files section for package generation again
- Turns out this change does more harm than good, people are actually
  relying on this behavior to avoid zillion conditionals in the spec.
  We'll want a saner way to conditionally disable/enable packages
  but until we have that, revert to the former longstanding behavior.
- This effectively reverts commit d010ec1c9026b35f5b2521be965c03c826894ce8