Refactor Extension loading (#627)

Deprecated naming support is removed:

* Removed special treatment for modules in `markdown.extensions`
* Removed support for `mdx_` prefixes.

Support for Entry Point names added:

Support for "short names" are now implemented with entry points. 
Therefore all the users who call extension names as `toc` will not 
get errors as the builtin extensions all have entry points defined 
which match the old "short names" for modules in 
`markdown.extensions`. The benefit is that any extension can offer
the same support without requiring the user to manually copy a file 
to that location on the file system (way to many extension authors 
have included such instructions in their installation documentation).

The one odd thing about this is that we have been issuing a 
DeprecationWarning for short names and now they are fully supported 
again. But I think it's the right thing to do.

Support for using dot notation is not removed. After all, it was never 
deprecated. And we shouldn't "force" entry points. There are plenty of 
reasons why users may not want that and not all of them can be 
resolved by using class instances instead.

All of the following ways to load an extension are valid:

    # Class instance
    from markdown.extensions.toc import TocExtension
    markdown.markdown(src, extensions=[TocExtension()]

    # Entry point name
    markdown.markdown(src, extensions=['toc'])

    # Dot notation with class
    markdown.markdown(src, extensions=['markdown.extensions.toc:TocExtension'])

    # Dot notation without class
    markdown.markdown(src, extensions=['markdown.extensions.toc'])

1 files changed, 6 insertions, 43 deletions

diff --git a/tests/test_apis.py b/tests/test_apis.py
index 6a1829b..42e7496 100644
--- a/tests/test_apis.py
+++ b/tests/test_apis.py
@@ -11,7 +11,6 @@ from __future__ import unicode_literals
 import unittest
 import sys
 import os
-import types
 import markdown
 import warnings
 from markdown.__main__ import parse_options
@@ -46,11 +45,15 @@ class TestMarkdownBasics(unittest.TestCase):
         from markdown.extensions.footnotes import FootnoteExtension
         markdown.Markdown(extensions=[FootnoteExtension()])
 
-    def testNamedExtension(self):
+    def testEntryPointExtension(self):
+        """ Test Extension loading with an entry point. """
+        markdown.Markdown(extensions=['footnotes'])
+
+    def testDotNotationExtension(self):
         """ Test Extension loading with Name (`path.to.module`). """
         markdown.Markdown(extensions=['markdown.extensions.footnotes'])
 
-    def TestNamedExtensionWithClass(self):
+    def TestDotNotationExtensionWithClass(self):
         """ Test Extension loading with class name (`path.to.module:Class`). """
         markdown.Markdown(extensions=['markdown.extensions.footnotes:FootnoteExtension'])
 
@@ -343,46 +346,6 @@ class TestErrors(unittest.TestCase):
             markdown.Markdown, extensions=[markdown.extensions.Extension()]
         )
 
-    def testMdxExtention(self):
-        """ Test that prepending mdx_ raises a DeprecationWarning. """
-        _create_fake_extension(name='fake', use_old_style=True)
-        self.assertRaises(
-            DeprecationWarning,
-            markdown.Markdown, extensions=['fake']
-        )
-
-    def testShortNameExtention(self):
-        """ Test that using a short name raises a DeprecationWarning. """
-        self.assertRaises(
-            DeprecationWarning,
-            markdown.Markdown, extensions=['footnotes']
-        )
-
-
-def _create_fake_extension(name, has_factory_func=True, is_wrong_type=False, use_old_style=False):
-    """ Create a fake extension module for testing. """
-    if use_old_style:
-        mod_name = '_'.join(['mdx', name])
-    else:
-        mod_name = name
-    if not PY3:
-        # mod_name must be bytes in Python 2.x
-        mod_name = bytes(mod_name)
-    ext_mod = types.ModuleType(mod_name)
-
-    def makeExtension(*args, **kwargs):
-        if is_wrong_type:
-            return object
-        else:
-            return markdown.extensions.Extension(*args, **kwargs)
-
-    if has_factory_func:
-        ext_mod.makeExtension = makeExtension
-    # Warning: this brute forces the extenson module onto the system. Either
-    # this needs to be specificly overriden or a new python session needs to
-    # be started to get rid of this. This should be ok in a testing context.
-    sys.modules[mod_name] = ext_mod
-
 
 class testETreeComments(unittest.TestCase):
     """