defaults: rename default.d to defaultrc to make it a valid package name

"default.d" is not a valid Python package name, and it needs to be a
valid package name if we want to be able to load the files from here
as resources.

Differential Revision: https://phab.mercurial-scm.org/D7426

mercurial/rcutil.py

@@ -60,9 +60,9 b' def envrcitems(env=None):'
 
 
 def defaultrcpath():
-    '''return rc paths in default.d'''
+    '''return rc paths in defaultrc'''
     path = []
-    defaultpath = os.path.join(util.datapath, b'default.d')
+    defaultpath = os.path.join(util.datapath, b'defaultrc')
     if os.path.isdir(defaultpath):
         path = _expandrcpath(defaultpath)
     return path