Move trie implementation out of plugins

We're going to use it for statistics, so it no longer belongs hidden in
the flake8.plugins namespace.

docs/source/internal/plugin_handling.rst

@@ -69,7 +69,7 @@ notified.
 To implement this goal, we needed an object to store listeners in that would
 allow for efficient look up - a Trie (or Prefix Tree). Given that none of the
 existing packages on PyPI allowed for storing data on each node of the trie,
-it was left up to write our own as :class:`~flake8.plugins._trie.Trie`. On
+it was left up to write our own as :class:`~flake8._trie.Trie`. On
 top of that we layer our :class:`~flake8.plugins.notifier.Notifier` class.
 
 Now when |Flake8| receives an error or warning, we can easily call the
@@ -122,7 +122,7 @@ API Documentation
 
 .. autoclass:: flake8.plugins.notifier.Notifier
 
-.. autoclass:: flake8.plugins._trie.Trie
+.. autoclass:: flake8._trie.Trie
 
 .. |PluginManager| replace:: :class:`~flake8.plugins.manager.PluginManager`
 .. |Plugin| replace:: :class:`~flake8.plugins.manager.Plugin`

src/flake8/plugins/notifier.py

@@ -1,5 +1,5 @@
 """Implementation of the class that registers and notifies listeners."""
-from flake8.plugins import _trie
+from flake8 import _trie
 
 
 class Notifier(object):

tests/unit/test_trie.py

@@ -1,5 +1,5 @@
 """Unit test for the _trie module."""
-from flake8.plugins import _trie as trie
+from flake8 import _trie as trie
 
 
 class TestTrie(object):