parser: move alias definition parser to common rule-set class
The original _parsealiasdefn() function is split into common _builddefn()
and revset-specific _parsealiasdefn(). revset._relabelaliasargs() is removed
as it is no longer used.
The doctests are ported by using the dummy parse().
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)