module: avoid JSON.stringify() for cache key

By avoiding JSON.stringify() and simply joining the strings with a
delimiter that does not appear in paths, we can improve cached
require() performance by at least 50%.

Additionally, this commit removes the last source of permanent
function deoptimization (const) for _findPath().

PR-URL: https://github.com/nodejs/node/pull/10789


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>