concretizer: simplify and suppress warnings for variant handling

We are relying on default logic in the variant handling in that we set a
default value if we never see `variant_set(P, V, X)`.

- Move the logic for this into `concretize.lp` instead of generating it
  for every package.

- For programs that don't have explicit variant settings, clingo warns
  that variant_set(P, V, X) doesn't appear in any rule head, because a
  setting is never generated.

  - Specifically suppress this warning.

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