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: nodejs#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>

lib/module.js

@@ -126,19 +126,18 @@ function tryExtensions(p, exts) {
 
 var warned = false;
 Module._findPath = function(request, paths) {
-  var exts = Object.keys(Module._extensions);
-
   if (path.isAbsolute(request)) {
     paths = [''];
   }
 
-  var trailingSlash = (request.slice(-1) === '/');
-
   var cacheKey = JSON.stringify({request: request, paths: paths});
   if (Module._pathCache[cacheKey]) {
     return Module._pathCache[cacheKey];
   }
 
+  const exts = Object.keys(Module._extensions);
+  const trailingSlash = request.slice(-1) === '/';
+
   // For each path
   for (var i = 0, PL = paths.length; i < PL; i++) {
     // Don't search further if path doesn't exist