packaging: include templates with their package as key in package_data

This is similar to an earlier patch in this series. It seems more
correct to use `mercurial.templates.coal` etc as keys in the
`package_data` dict now that those modules are packages.

Differential Revision: https://phab.mercurial-scm.org/D8858

setup.py

@@ -1635,10 +1635,8 b' def ordinarypath(p):'
 
 for root in ('templates',):
     for curdir, dirs, files in os.walk(os.path.join('mercurial', root)):
-        curdir = curdir.split(os.sep, 1)[1]
-        for f in filter(ordinarypath, files):
-            f = os.path.join(curdir, f)
-            packagedata['mercurial'].append(f)
+        packagename = curdir.replace(os.sep, '.')
+        packagedata[packagename] = list(filter(ordinarypath, files))
 
 datafiles = []