Cleanups to the common.sys_info API

* Move get_all_subclasses out of sys_info as it is unrelated to system
  information.
* get_all_subclasses now returns a set() instead of a list.
* Don't port get_platform to sys_info as it is deprecated.  Code using
  the common API should just use platform.system() directly.
* Rename load_platform_subclass() to get_platform_subclass and do not
  instantiate the rturned class.
* Test the compat shims in module_utils/basic.py separately from the new
  API in module_utils/common/sys_info.py and module_utils/common/_utils.py

lib/ansible/module_utils/common/_utils.py

@@ -0,0 +1,40 @@
+# Copyright (c) 2018, Ansible Project
+# Simplified BSD License (see licenses/simplified_bsd.txt or https://opensource.org/licenses/BSD-2-Clause)
+
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+
+"""
+Modules in _utils are waiting to find a better home.  If you need to use them, be prepared for them
+to move to a different location in the future.
+"""
+
+
+def get_all_subclasses(cls):
+    '''
+    Recursively search and find all subclasses of a given class
+
+    :arg cls: A python class
+    :rtype: set
+    :returns: The set of python classes which are the subclasses of `cls`.
+
+    In python, you can use a class's :py:meth:`__subclasses__` method to determine what subclasses
+    of a class exist.  However, `__subclasses__` only goes one level deep.  This function searches
+    each child class's `__subclasses__` method to find all of the descendent classes.  It then
+    returns an iterable of the descendent classes.
+    '''
+    # Retrieve direct subclasses
+    subclasses = set(cls.__subclasses__())
+    to_visit = list(subclasses)
+    # Then visit all subclasses
+    while to_visit:
+        for sc in to_visit:
+            # The current class is now visited, so remove it from list
+            to_visit.remove(sc)
+            # Appending all subclasses to visit and keep a reference of available class
+            for ssc in sc.__subclasses__():
+                if ssc not in subclasses:
+                    to_visit.append(ssc)
+                    subclasses.add(ssc)
+    return subclasses