templates: add __init__.py files to templates/ dirs This is necessary for them to be loaded with `importlib.resources`, which we want to do for PyOxidizer and similar. `importlib.resources` cannot read resources from submodules (`resources.open_binary('mercurial.templates', 'coal/map')` is not valid), so we need one `__init__.py` per directory. Differential Revision: https://phab.mercurial-scm.org/D8854