Avoid rewrite warning for inline runs

When running pytest inline/inprocess we plugins have already been
imported and re-writen, so avoid the warning.

_pytest/assertion/rewrite.py

@@ -163,9 +163,9 @@ class AssertionRewritingHook(object):
                     self.session = session
                     del session
         else:
-            for marked in self._must_rewrite:
-                if marked.startswith(name):
-                    return True
+            toplevel_name = name.split('.', 1)[0]
+            if toplevel_name in self._must_rewrite:
+                return True
         return False
 
     def mark_rewrite(self, *names):