concretizer: unify handling of single- and multi-valued variants
concretizer: unify handling of single- and multi-valued variants

Multi-valued variants previously maximized default values to handle
cases where the default contained two values, e.g.:

    variant("foo", default="bar,baz")

This is because previously we were minimizing non-default values, and
`foo=bar`, `foo=baz`, and `foo=bar,baz` all had the same score, as
none of them had any "non-default" values.

This commit changes the approach and considers a non-default value
to be either a value set to something not default *or* the absence
of a default value from the set value.  This allows multi- and
single-valued variants to be handled the same way, with the same
minimization criterion.  It alse means that the "best" value for every
optimization criterion is now zero, which allows us to make useful
assumptions about the optimization criteria.

LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack