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/unix/users.cc

@@ -9,6 +9,8 @@
 
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 std::string getUserName()
 {
     auto pw = getpwuid(geteuid());