policy: extend API version checks for cffi

This is just a stub for future extension. I could add a version constant to
CFFI modules by putting it to both ffi.set_source() and ffi.cdef(), but that
doesn't seem right. So for now, cffi modules will be explicitly unversioned
(i.e. version constant must be undefined or set to None.) We can revisit it
later when we need to consider CFFI support more seriously.

mercurial/policy.py

@@ -72,16 +72,16 b' def _importfrom(pkgname, modname):'
 
 # keep in sync with "version" in C modules
 _cextversions = {
-    r'base85': 1,
-    r'bdiff': 1,
-    r'diffhelpers': 1,
-    r'mpatch': 1,
-    r'osutil': 1,
-    r'parsers': 1,
+    (r'cext', r'base85'): 1,
+    (r'cext', r'bdiff'): 1,
+    (r'cext', r'diffhelpers'): 1,
+    (r'cext', r'mpatch'): 1,
+    (r'cext', r'osutil'): 1,
+    (r'cext', r'parsers'): 1,
 }
 
 def _checkmod(pkgname, modname, mod):
-    expected = _cextversions.get(modname)
+    expected = _cextversions.get((pkgname, modname))
     actual = getattr(mod, r'version', None)
     if actual != expected:
         raise ImportError(r'cannot import module %s.%s '