Multi-valued variants: better support for combinations (#9481)

This enforces conventions that allow for correct handling of
multi-valued variants where specifying no value is an option,
and adds convenience functionality for specifying multi-valued
variants with conflicting sets of values. This also adds a notion
of "feature values" for variants, which are those that are understood
by the build system (e.g. those that would appear as configure
options). In more detail:

* Add documentation on variants to the packaging guide
* Forbid usage of '' or None as a possible variant value, in
  particular as a default. To indicate choosing no value, the user
  must explicitly define an option like 'none'. Without this,
  multi-valued variants with default set to None were not parsable
  from the command line (Fixes #6314)
* Add "disjoint_sets" function to support the declaration of
  multi-valued variants with conflicting sets of options. For example
  a variant "foo" with possible values "a", "b", and "c" where "c"
  is exclusive of the other values ("foo=a,b" and "foo=c" are
  valid but "foo=a,c" is not).
* Add "any_combination_of" function to support the declaration of
  multi-valued variants where it is valid to choose none of the
  values. This automatically defines "none" as an option (exclusive
  with all other choices); this value does not appear when iterating
  over the variant's values, for example in "with_or_without" (which
  constructs autotools option strings from variant values).
* The "disjoint_sets" and "any_combination_of" methods return an
  object which tracks the possible values. It is also possible to
  indicate that some of these values do not correspond to options
  understood by the package's build system, such that methods like
  "with_or_without" will not define options for those values (this
  occurs automatically for "none")
* Add documentation for usage of new functions for specifying
  multi-valued variants

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