More support for std::filepath in libnixutil

We're not replacing `Path` in exposed definitions in many cases, but
just adding alternatives. This will allow us to "top down" change `Path`
to `std::fileysystem::path`, and then we can remove the `Path`-using
utilities which will become unused.

Also add some test files which we forgot to include in the libutil unit
tests `meson.build`.

Co-Authored-By: siddhantCodes <siddhantk232@gmail.com>

src/libutil/executable-path.hh

@@ -7,11 +7,15 @@ namespace nix {
 
 MakeError(ExecutableLookupError, Error);
 
+/**
+ * @todo rename, it is not just good for execuatable paths, but also
+ * other lists of paths.
+ */
 struct ExecutablePath
 {
     std::vector<std::filesystem::path> directories;
 
-    constexpr static const OsString::value_type separator =
+    constexpr static const OsChar separator =
 #ifdef WIN32
         L';'
 #else