* Add missing MultiMethodMeta metaclass in builders
and remove the Python 2 fallback option in favor of hard errors to catch
similar issues going forward.
The fallback option can cause about 10K stat calls due to use of
`realpath` in the inspect module, depending on how deep Spack itself is
nested in the file system, which is ... undesirable.
* code shuffling to avoid circular import
* more reshuffling
* move reserved variant names into variants module
d40f847497