mercurial: be more strict about loading dual implemented modules

With this change in place, we should have slightly stronger guarantees
about how modules with both Python and C implementations are loaded.
Before, our module loader's default policy looked under both mercurial/*
and mercurial/pure/* and imported whatever it found, C or pure. The fact
it looked in both locations by default was a temporary regression from
the beginning of this series.

This patch does 2 things:

1) Changes the default module load policy to only load C modules
2) Verifies that files loaded from mercurial/* are actually C modules

This 2nd behavior change makes our new module loading mechanism
stricter than from before this series. Before, it was possible to load
a .py-based module from mercurial/*. This could happen if an old
installation orphaned a file and then somehow didn't install the C
version for the new install. We now detect this odd configuration
and fall back to loading the pure Python module, assuming it is
allowed. In the case of a busted installation, we fail fast. While
we could fall back, we explicitly decide not to do this because
we don't want people accidentally not running the C modules and having
slow performance as a result.

mercurial/__init__.py

@@ -22,8 +22,7 b" modulepolicy = '@MODULELOADPOLICY@'"
 
 # By default, require the C extensions for performance reasons.
 if modulepolicy == '@' 'MODULELOADPOLICY' '@':
-    # TODO change to 'c' once installer is changed.
-    modulepolicy = 'allow'
+    modulepolicy = 'c'
 
 # Environment variable can always force settings.
 modulepolicy = os.environ.get('HGMODULEPOLICY', modulepolicy)
@@ -79,11 +78,9 b' class hgimporter(object):'
             # mercurial/* are C extensions. If the current policy allows the
             # loading of .py modules, the module will be re-imported from
             # mercurial/pure/* below.
-            # TODO uncomment once setup.py is updated to actually install
-            # into mercurial/pure.
-            #if modinfo[2][2] != imp.C_EXTENSION:
-            #    raise ImportError('.py version of %s found where C '
-            #                      'version should exist' % name)
+            if modinfo[2][2] != imp.C_EXTENSION:
+                raise ImportError('.py version of %s found where C '
+                                  'version should exist' % name)
 
         except ImportError:
             if modulepolicy == 'c':