refactor(3p/nix): Replace tokenizeStrings with absl::StrSplit

This function was a custom (and inefficient in the case of
single-character delimiters) string splitter which was used all over
the codebase. Abseil provides an appropriate replacement function.

third_party/nix/src/libutil/util.hh

@@ -56,7 +56,7 @@ Path canonPath(const Path& path, bool resolveSymlinks = false);
    everything before the final `/'.  If the path is the root or an
    immediate child thereof (e.g., `/foo'), this means an empty string
    is returned. */
-Path dirOf(const Path& path);
+Path dirOf(absl::string_view path);
 
 /* Return the base name of the given canonical path, i.e., everything
    following the final `/'. */
@@ -316,11 +316,6 @@ MakeError(Interrupted, BaseError);
 
 MakeError(FormatError, Error);
 
-/* String tokenizer. */
-template <class C>
-C tokenizeString(const std::string& s,
-                 const std::string& separators = " \t\n\r");
-
 /* Concatenate the given strings with a separator between the
    elements. */
 std::string concatStringsSep(const std::string& sep, const Strings& ss);