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/os-string.hh

@@ -0,0 +1,43 @@
+#pragma once
+///@file
+
+#include <optional>
+#include <string>
+#include <string_view>
+
+namespace nix {
+
+/**
+ * Named because it is similar to the Rust type, except it is in the
+ * native encoding not WTF-8.
+ *
+ * Same as `std::filesystem::path::string_type`, but manually defined to
+ * avoid including a much more complex header.
+ */
+using OsString = std::basic_string<
+#if defined(_WIN32) && !defined(__CYGWIN__)
+    wchar_t
+#else
+    char
+#endif
+    >;
+
+/**
+ * `std::string_view` counterpart for `OsString`.
+ */
+using OsStringView = std::basic_string_view<OsString::value_type>;
+
+std::string os_string_to_string(OsStringView path);
+
+OsString string_to_os_string(std::string_view s);
+
+/**
+ * Create string literals with the native character width of paths
+ */
+#ifndef _WIN32
+#  define OS_STR(s) s
+#else
+#  define OS_STR(s) L##s
+#endif
+
+}