Unexpose config headers (low hanging fruit only)

- Some headers were completely redundant and have been removed.
- Other headers have been turned private.
- Unnecessary meson.build code has been removed.
- libutil-tests now has a private config header, where previously
  it had none. This removes the need to expose a package version
  macro publicly.

tests/functional/plugins/meson.build

@@ -6,7 +6,6 @@ libplugintest = shared_module(
     # It would be nice for our headers to be idempotent instead.
     '-include', 'config-util.hh',
     '-include', 'config-store.hh',
-    # '-include', 'config-fetchers.hh',
     '-include', 'config-expr.hh',
   ],
   dependencies : [