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/file-path.hh

@@ -1,10 +1,10 @@
 #pragma once
 ///@file
 
-#include <optional>
 #include <filesystem>
 
 #include "types.hh"
+#include "os-string.hh"
 
 namespace nix {
 
@@ -22,39 +22,26 @@ typedef std::set<std::filesystem::path> PathSetNG;
  *
  * @todo drop `NG` suffix and replace the one in `types.hh`.
  */
-struct PathViewNG : std::basic_string_view<std::filesystem::path::value_type>
+struct PathViewNG : OsStringView
 {
-    using string_view = std::basic_string_view<std::filesystem::path::value_type>;
+    using string_view = OsStringView;
 
     using string_view::string_view;
 
     PathViewNG(const std::filesystem::path & path)
-        : std::basic_string_view<std::filesystem::path::value_type>(path.native())
+        : OsStringView{path.native()}
     { }
 
-    PathViewNG(const std::filesystem::path::string_type & path)
-        : std::basic_string_view<std::filesystem::path::value_type>(path)
+    PathViewNG(const OsString & path)
+        : OsStringView{path}
     { }
 
     const string_view & native() const { return *this; }
     string_view & native() { return *this; }
 };
 
-std::string os_string_to_string(PathViewNG::string_view path);
-
-std::filesystem::path::string_type string_to_os_string(std::string_view s);
-
 std::optional<std::filesystem::path> maybePath(PathView path);
 
 std::filesystem::path pathNG(PathView path);
 
-/**
- * Create string literals with the native character width of paths
- */
-#ifndef _WIN32
-# define PATHNG_LITERAL(s) s
-#else
-# define PATHNG_LITERAL(s) L ## s
-#endif
-
 }