policy: eliminate ".pure." from module name only if marked as dual

So we can switch cext/pure modules to new layout one by one.

contrib/check-py3-compat.py

@@ -14,6 +14,16 b' import os'
 import sys
 import traceback
 
+# Modules that have both Python and C implementations.
+_dualmodules = (
+    'base85.py',
+    'bdiff.py',
+    'diffhelpers.py',
+    'mpatch.py',
+    'osutil.py',
+    'parsers.py',
+)
+
 def check_compat_py2(f):
     """Check Python 3 compatibility for a file with Python 2"""
     with open(f, 'rb') as fh:
@@ -55,7 +65,9 b' def check_compat_py3(f):'
     # out module paths for things not in a package can be confusing.
     if f.startswith(('hgext/', 'mercurial/')) and not f.endswith('__init__.py'):
         assert f.endswith('.py')
-        name = f.replace('/', '.')[:-3].replace('.pure.', '.')
+        name = f.replace('/', '.')[:-3]
+        if f.endswith(_dualmodules):
+            name = name.replace('.pure.', '.')
         try:
             importlib.import_module(name)
         except Exception as e:

contrib/import-checker.py

@@ -24,6 +24,16 b' allowsymbolimports = ('
     'mercurial.node',
 )
 
+# Modules that have both Python and C implementations.
+_dualmodules = (
+    'base85.py',
+    'bdiff.py',
+    'diffhelpers.py',
+    'mpatch.py',
+    'osutil.py',
+    'parsers.py',
+)
+
 # Modules that must be aliased because they are commonly confused with
 # common variables and can create aliasing and readability issues.
 requirealias = {
@@ -691,7 +701,8 b' def main(argv):'
     used_imports = {}
     any_errors = False
     for source_path in argv[1:]:
-        modname = dotted_name_of_path(source_path, trimpure=True)
+        trimpure = source_path.endswith(_dualmodules)
+        modname = dotted_name_of_path(source_path, trimpure=trimpure)
         localmods[modname] = source_path
     for localmodname, source_path in sorted(localmods.items()):
         for src, modname, name, line in sources(source_path, localmodname):

mercurial/__init__.py

@@ -165,7 +165,8 b' if sys.version_info[0] >= 3:'
             if not spec:
                 return None
 
-            if fullname.startswith('mercurial.pure.'):
+            if (fullname.startswith('mercurial.pure.')
+                and fullname.replace('.pure.', '.') in _dualmodules):
                 spec.name = spec.name.replace('.pure.', '.')
 
             # TODO need to support loaders from alternate specs, like zip