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.

src/libutil-c/nix_api_util.cc

@@ -7,6 +7,8 @@
 #include <cxxabi.h>
 #include <typeinfo>
 
+#include "config-util.h"
+
 nix_c_context * nix_c_context_create()
 {
     return new nix_c_context();