importer: new format to signal ImportError

Previously we'd send just None in GET_MODULE reply, but now since there
is no single request-reply structure, we must include the fullname in
the LOAD_MODULE response and make all of its data fields None to
indicate the same.

mitogen/core.py

@@ -520,7 +520,7 @@ class Importer(object):
         event.wait()
 
         ret = self._cache[fullname]
-        if ret is None:
+        if ret[2] is None:
             raise ImportError('Master does not have %r' % (fullname,))
 
         pkg_present = ret[1]

mitogen/master.py

@@ -635,6 +635,12 @@ class ModuleResponder(object):
             return self._cache[fullname]
 
         path, source, is_pkg = self._finder.get_module_source(fullname)
+        if source is None:
+            LOG.error('_build_tuple(%r): could not locate source', fullname)
+            tup = fullname, None, None, None, None
+            self._cache[fullname] = tup
+            return tup
+
         if source is None:
             raise ImportError('could not find %r' % (fullname,))
 
@@ -675,8 +681,7 @@ class ModuleResponder(object):
 
         try:
             tup = self._build_tuple(fullname)
-
-            for name in tup[4]:  # related
+            for name in tup[4] or ():  # related
                 if name == fullname:
                     # Must be sent last
                     continue