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-system.hh

@@ -263,6 +263,12 @@ std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
  */
 Path defaultTempDir();
 
+/**
+ * Interpret `exe` as a location in the ambient file system and return
+ * whether it exists AND is executable.
+ */
+bool isExecutableFileAmbient(const std::filesystem::path & exe);
+
 /**
  * Used in various places.
  */