path: refactor code for clarity

This moves a condition inside of a for loop which can only be
triggered at the very end of the for loop outside of the loop. That
way the for loop itself is much simpler and easier to understand and
the code itself is less indented which should increase the
readability.

It also refactors some `var` to `let` and `const`.

PR-URL: https://github.com/nodejs/node/pull/25278


Reviewed-By: Michaël Zasso <targos@protonmail.com>