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

test/units/module_utils/common/test_utils.py

@@ -0,0 +1,46 @@
+# -*- coding: utf-8 -*-
+# (c) 2018 Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+from ansible.module_utils.common.sys_info import get_all_subclasses
+
+
+#
+# Tests for get_all_subclasses
+#
+
+class TestGetAllSubclasses:
+    class Base:
+        pass
+
+    class BranchI(Base):
+        pass
+
+    class BranchII(Base):
+        pass
+
+    class BranchIA(BranchI):
+        pass
+
+    class BranchIB(BranchI):
+        pass
+
+    class BranchIIA(BranchII):
+        pass
+
+    class BranchIIB(BranchII):
+        pass
+
+    def test_bottom_level(self):
+        assert get_all_subclasses(self.BranchIIB) == set()
+
+    def test_one_inheritance(self):
+        assert set(get_all_subclasses(self.BranchII)) == set([self.BranchIIA, self.BranchIIB])
+
+    def test_toplevel(self):
+        assert set(get_all_subclasses(self.Base)) == set([self.BranchI, self.BranchII,
+                                                          self.BranchIA, self.BranchIB,
+                                                          self.BranchIIA, self.BranchIIB])