packaging: make "mercurial.defaultrc" a key in package_data

Before this patch, we had a `mercurial` key with a `defaultrc/*.rc`
value. It seems more correct to have a `mercurial.defaultrc` key with
a `*.rc` value since `mercurial.defaultrc` it became a pacakge in
1390bb81163e (help: get helptext/ data from `resources` module if
available, 2019-12-12).

Differential Revision: https://phab.mercurial-scm.org/D8856

setup.py

@@ -1614,11 +1614,8 b" if os.name == 'nt':"
     msvccompiler.MSVCCompiler = HackedMSVCCompiler
 
 packagedata = {
-    'mercurial': [
-        'locale/*/LC_MESSAGES/hg.mo',
-        'defaultrc/*.rc',
-        'dummycert.pem',
-    ],
+    'mercurial': ['locale/*/LC_MESSAGES/hg.mo', 'dummycert.pem',],
+    'mercurial.defaultrc': ['*.rc',],
     'mercurial.helptext': ['*.txt',],
     'mercurial.helptext.internals': ['*.txt',],
 }