Factor out lookupExecutable and other PATH improvments

This ended up motivating a good deal of other infra improvements in
order to get Windows right:

- `OsString` to complement `std::filesystem::path`

- env var code for working with the underlying `OsString`s

- Rename `PATHNG_LITERAL` to `OS_STR`

- `NativePathTrait` renamed to `OsPathTrait`, given a character template
  parameter until #9205 is complete.

Split `tests.cc` matching split of `util.{cc,hh}` last year.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/unix/file-path.cc

@@ -8,16 +8,6 @@
 
 namespace nix {
 
-std::string os_string_to_string(PathViewNG::string_view path)
-{
-    return std::string { path };
-}
-
-std::filesystem::path::string_type string_to_os_string(std::string_view s)
-{
-    return std::string { s };
-}
-
 std::optional<std::filesystem::path> maybePath(PathView path)
 {
     return { path };