Use namespace-aware meta importer for ipaplatform

Instead of symlinks and build-time configuration the ipaplatform module
is now able to auto-detect platforms on import time. The meta importer
uses the platform 'ID' from /etc/os-releases. It falls back to 'ID_LIKE'
on platforms like CentOS, which has ID=centos and ID_LIKE="rhel fedora".

The meta importer is able to handle namespace packages and the
ipaplatform package has been turned into a namespace package in order to
support external platform specifications.

https://fedorahosted.org/freeipa/ticket/6474

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipaplatform/constants.py

@@ -0,0 +1,8 @@
+#
+# Copyright (C) 2017  FreeIPA Contributors see COPYING for license
+#
+"""IpaMetaImporter replaces this module with ipaplatform.$NAME.constants.
+"""
+import ipaplatform._importhook
+
+ipaplatform._importhook.fixup_module('ipaplatform.constants')