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/meson.build

@@ -1,4 +1,4 @@
-subdir('mm')
-subdir('dummy')
-subdir('ofono')
-subdir('sip')
+subdir('provider/mm')
+subdir('provider/dummy')
+subdir('provider/ofono')
+subdir('provider/sip')