Move provider plugins into a dedicated directory

Since we will introduce another type of plugin for the policy engine
we want to have each plugin type in separate directories.

We also have to adjust:

- plugin search directories
- po file location
- update paths for calls-doc target

plugins/provider/dummy/dummy.plugin.in

@@ -0,0 +1,7 @@
+[Plugin]
+Module=dummy
+Name=Dummy
+Description=Dummy calls provider
+Authors=Bob Ham <rah@settrans.net>
+Copyright=Copyright (C) 2018 Purism SPC
+Website=@PACKAGE_URL_RAW@