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/libexpr-c/meson.build

@@ -14,8 +14,6 @@ cxx = meson.get_compiler('cpp')
 
 subdir('nix-meson-build-support/deps-lists')
 
-configdata = configuration_data()
-
 deps_private_maybe_subproject = [
   dependency('nix-util'),
   dependency('nix-store'),
@@ -27,14 +25,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('nix-meson-build-support/subprojects')
 
-# TODO rename, because it will conflict with downstream projects
-configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
-
-config_h = configure_file(
-  configuration : configdata,
-  output : 'config-expr.h',
-)
-
 add_project_arguments(
   # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
   # It would be nice for our headers to be idempotent instead.
@@ -44,10 +34,6 @@ add_project_arguments(
   '-include', 'config-store.hh',
   '-include', 'config-expr.hh',
 
-  # From C libraries, for our public, installed headers too
-  '-include', 'config-util.h',
-  '-include', 'config-store.h',
-  '-include', 'config-expr.h',
   language : 'cpp',
 )
 
@@ -61,7 +47,7 @@ sources = files(
 
 include_dirs = [include_directories('.')]
 
-headers = [config_h] + files(
+headers = files(
   'nix_api_expr.h',
   'nix_api_external.h',
   'nix_api_value.h',