import-checker: allow importing symbols from pure modules

This allows us to re-export pure functions from cffi modules:

  # mercurial/cffi/base85.py
  from ..pure.base85 import *

contrib/import-checker.py

@@ -22,6 +22,13 b' allowsymbolimports = ('
     'mercurial.hgweb.request',
     'mercurial.i18n',
     'mercurial.node',
+    # for cffi modules to re-export pure functions
+    'mercurial.pure.base85',
+    'mercurial.pure.bdiff',
+    'mercurial.pure.diffhelpers',
+    'mercurial.pure.mpatch',
+    'mercurial.pure.osutil',
+    'mercurial.pure.parsers',
 )
 
 # Whitelist of symbols that can be directly imported.