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_sys_info.py

@@ -0,0 +1,108 @@
+# -*- coding: utf-8 -*-
+# (c) 2012-2014, Michael DeHaan <michael.dehaan@gmail.com>
+# (c) 2016 Toshio Kuratomi <tkuratomi@ansible.com>
+# (c) 2017-2018 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
+
+import pytest
+
+from units.compat.mock import patch
+
+from ansible.module_utils.six.moves import builtins
+
+# Functions being tested
+from ansible.module_utils.common.sys_info import get_distribution
+from ansible.module_utils.common.sys_info import get_distribution_version
+from ansible.module_utils.common.sys_info import get_platform_subclass
+
+
+realimport = builtins.__import__
+
+
+@pytest.fixture
+def platform_linux(mocker):
+    mocker.patch('platform.system', return_value='Linux')
+
+
+#
+# get_distribution tests
+#
+
+def test_get_distribution_not_linux():
+    """If it's not Linux, then it has no distribution"""
+    with patch('platform.system', return_value='Foo'):
+        assert get_distribution() is None
+
+
+@pytest.mark.usefixtures("platform_linux")
+class TestGetDistribution:
+    """ Tests for get_distribution that have to find somethine"""
+    def test_distro_known(self):
+        with patch('ansible.module_utils.distro.name', return_value="foo"):
+            assert get_distribution() == "Foo"
+
+    def test_distro_unknown(self):
+        with patch('ansible.module_utils.distro.name', return_value=""):
+            assert get_distribution() == "OtherLinux"
+
+    def test_distro_amazon_part_of_another_name(self):
+        with patch('ansible.module_utils.distro.name', return_value="AmazonFooBar"):
+            assert get_distribution() == "Amazonfoobar"
+
+    def test_distro_amazon_linux(self):
+        with patch('ansible.module_utils.distro.name', return_value="Amazon Linux AMI"):
+            assert get_distribution() == "Amazon"
+
+
+#
+# get_distribution_version tests
+#
+
+def test_get_distribution_version_not_linux():
+    """If it's not Linux, then it has no distribution"""
+    with patch('platform.system', return_value='Foo'):
+        assert get_distribution_version() is None
+
+
+@pytest.mark.usefixtures("platform_linux")
+def test_distro_found():
+    with patch('ansible.module_utils.distro.version', return_value="1"):
+        assert get_distribution_version() == "1"
+
+
+#
+# Tests for get_platform_subclass
+#
+
+class TestGetPlatformSubclass:
+    class LinuxTest:
+        pass
+
+    class Foo(LinuxTest):
+        platform = "Linux"
+        distribution = None
+
+    class Bar(LinuxTest):
+        platform = "Linux"
+        distribution = "Bar"
+
+    def test_not_linux(self):
+        # if neither match, the fallback should be the top-level class
+        with patch('platform.system', return_value="Foo"):
+            with patch('ansible.module_utils.common.sys_info.get_distribution', return_value=None):
+                assert get_platform_subclass(self.LinuxTest) is self.LinuxTest
+
+    @pytest.mark.usefixtures("platform_linux")
+    def test_get_distribution_none(self):
+        # match just the platform class, not a specific distribution
+        with patch('ansible.module_utils.common.sys_info.get_distribution', return_value=None):
+            assert get_platform_subclass(self.LinuxTest) is self.Foo
+
+    @pytest.mark.usefixtures("platform_linux")
+    def test_get_distribution_found(self):
+        # match both the distribution and platform class
+        with patch('ansible.module_utils.common.sys_info.get_distribution', return_value="Bar"):
+            assert get_platform_subclass(self.LinuxTest) is self.Bar