module: move unnecessary work for early return

The exts and trailingSlash variables are only used if the
path isn't cached. This commit moves them further down in the
code, and changes from var to const.

PR-URL: https://github.com/nodejs/node/pull/3579


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>