* 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
