contrib: tweak import-checker to always use bytes for module names

We could go the other route and make embedded() return this as a
bytes, but then it would be harder to portably doctest, so I prefer
this option.

Differential Revision: https://phab.mercurial-scm.org/D5206

contrib/import-checker.py

@@ -696,7 +696,7 b' def sources(f, modname):'
     if py or f.endswith('.t'):
         with open(f, 'rb') as src:
             for script, modname, t, line in embedded(f, modname, src):
-                yield script, modname, t, line
+                yield script, modname.encode('utf8'), t, line
 
 def main(argv):
     if len(argv) < 2 or (argv[1] == '-' and len(argv) > 2):