Add missing MultiMethodMeta metaclass in builders (#45879)

* 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

lib/spack/llnl/util/lang.py

@@ -84,20 +84,6 @@ def index_by(objects, *funcs):
     return result
 
 
-def caller_locals():
-    """This will return the locals of the *parent* of the caller.
-    This allows a function to insert variables into its caller's
-    scope.  Yes, this is some black magic, and yes it's useful
-    for implementing things like depends_on and provides.
-    """
-    # Passing zero here skips line context for speed.
-    stack = inspect.stack(0)
-    try:
-        return stack[2][0].f_locals
-    finally:
-        del stack
-
-
 def attr_setdefault(obj, name, value):
     """Like dict.setdefault, but for objects."""
     if not hasattr(obj, name):