import-checker: exclude mercurial packages installed into the system path

If mercurial was installed into a directory other than the site-packages,
test-module-imports.t failed as 'mercurial.node' was listed in stdlib_modules:

  testpackage/latesymbolimport.py relative import of stdlib module

Instead, we should exclude our packages explicitly.

contrib/import-checker.py

@@ -192,11 +192,10 b' def list_stdlib_modules():'
         # of any().
         if not any(libpath.startswith(p) for p in stdlib_prefixes): # no-py24
             continue
-        if 'site-packages' in libpath:
-            continue
         for top, dirs, files in os.walk(libpath):
             for i, d in reversed(list(enumerate(dirs))):
-                if not os.path.exists(os.path.join(top, d, '__init__.py')):
+                if (not os.path.exists(os.path.join(top, d, '__init__.py'))
+                    or top == libpath and d in ('hgext', 'mercurial')):
                     del dirs[i]
             for name in files:
                 if name == '__init__.py':