layouts: refactor out svn path to mercurial branch logic This pulls the logic for mapping from svn path to mercurial branch name out of svnmeta.py and into the new layouts library. It also sets up the structure for that library. This diff does not modify any call to svnmeta.localname, rather leaving it in place as a simple proxy to the new layout object.

include Makefile hgsubversion/help/*.rst
recursive-include tests *.py *.sh *.svndump *.txt