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.

(cherry picked from commit b86a76044e)

src/libexpr-tests/meson.build

@@ -41,9 +41,6 @@ add_project_arguments(
   '-include', 'config-util.hh',
   '-include', 'config-store.hh',
   '-include', 'config-expr.hh',
-  '-include', 'config-util.h',
-  '-include', 'config-store.h',
-  '-include', 'config-expr.h',
   language : 'cpp',
 )