Prove that PluginManger.register() ignores non-routines

This proves that pytest-dev#91 is a problem in `pytest` and not `pluggy`.
Thanks to @RonnyPfannschmidt for the fix about a year ago ;)

testing/test_pluginmanager.py

@@ -1,4 +1,5 @@
 import pytest
+import imp
 
 from pluggy import (PluginValidationError,
                     HookCallError, HookimplMarker, HookspecMarker)
@@ -350,3 +351,20 @@ def m(self, __multicall__, x):
 def test_add_hookspecs_nohooks(pm):
     with pytest.raises(ValueError):
         pm.add_hookspecs(10)
+
+
+def test_reject_prefixed_module(pm):
+    """Verify that a module type attribute that contains the project
+    prefix in its name (in this case `'example_*'` isn't collected
+    when registering a module which imports it.
+    """
+    pm._implprefix = 'example'
+    conftest = imp.new_module("conftest")
+    # note project prefix for ``pm`` is 'example'
+    conftest.example_blah = imp.new_module("example_blah")
+
+    # import pdb; pdb.set_trace()
+    name = pm.register(conftest)
+    assert name == 'conftest'
+    assert getattr(pm.hook, 'example_blah', None) is None
+    assert pm.parse_hookimpl_opts(conftest, 'example_blah') is None