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>

tests/unit/libutil/meson.build

@@ -48,12 +48,14 @@ subdir('build-utils-meson/diagnostics')
 sources = files(
   'args.cc',
   'canon-path.cc',
+  'checked-arithmetic.cc',
   'chunked-vector.cc',
   'closure.cc',
   'compression.cc',
   'config.cc',
   'executable-path.cc',
   'file-content-address.cc',
+  'file-system.cc',
   'git.cc',
   'hash.cc',
   'hilite.cc',
@@ -62,6 +64,7 @@ sources = files(
   'lru-cache.cc',
   'nix_api_util.cc',
   'pool.cc',
+  'position.cc',
   'processes.cc',
   'references.cc',
   'spawn.cc',