rename: build-utils-meson -> nix-meson-build-support

Fix a footgun. In my case, I had a couple of build ("output")
directories sitting around.

    rm -rf build-*

Was confused for a bit why a meson.build file was missing.

Probably also helps with autocompletion.

I tried meson-build-support first, but I had to add something like
a nix- prefix, in order to make meson happy. They've reserved the
meson- prefix.

src/libutil-tests/build-utils-meson

@@ -1 +0,0 @@
-../../build-utils-meson

src/libutil-tests/meson.build

@@ -14,7 +14,7 @@ project('nix-util-tests', 'cpp',
 
 cxx = meson.get_compiler('cpp')
 
-subdir('build-utils-meson/deps-lists')
+subdir('nix-meson-build-support/deps-lists')
 
 deps_private_maybe_subproject = [
   dependency('nix-util'),
@@ -23,10 +23,10 @@ deps_private_maybe_subproject = [
 ]
 deps_public_maybe_subproject = [
 ]
-subdir('build-utils-meson/subprojects')
+subdir('nix-meson-build-support/subprojects')
 
-subdir('build-utils-meson/export-all-symbols')
-subdir('build-utils-meson/windows-version')
+subdir('nix-meson-build-support/export-all-symbols')
+subdir('nix-meson-build-support/windows-version')
 
 rapidcheck = dependency('rapidcheck')
 deps_private += rapidcheck
@@ -42,7 +42,7 @@ add_project_arguments(
   language : 'cpp',
 )
 
-subdir('build-utils-meson/common')
+subdir('nix-meson-build-support/common')
 
 sources = files(
   'args.cc',

src/libutil-tests/nix-meson-build-support

@@ -0,0 +1 @@
+../../nix-meson-build-support

src/libutil-tests/package.nix

@@ -26,8 +26,8 @@ mkMesonExecutable (finalAttrs: {
 
   workDir = ./.;
   fileset = fileset.unions [
-    ../../build-utils-meson
-    ./build-utils-meson
+    ../../nix-meson-build-support
+    ./nix-meson-build-support
     ../../.version
     ./.version
     ./meson.build