parser: move functions that process alias expansion to rule-set class
They will be commonly used by revset and templater. It isn't easy to understand
how _expand() works, so I'll add comments by a follow-up patch.
The local variable 'alias' is renamed to 'a' to avoid shadowing the global
'alias' class.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])