module: skip directories known not to exist

There is no point in trying to search for files in a directory that
we know does not exist, so stop doing that.

Reduces the total number of stat(2) calls and the number of stat(2)
misses on a medium-sized application by about 21% and 29% respectively.

Reduces the total number of package.json open(2) calls and the number
of open(2) misses by about 21% and 93% (!) respectively.

Before:

    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     50.93    0.178419          38      4702           lstat
     29.08    0.101875          36      2800      2010 stat
     11.36    0.039796          43       932       215 open
      5.39    0.018897          34       550           fstat
      3.24    0.011337          34       336           pread
    ------ ----------- ----------- --------- --------- ----------------
    100.00    0.350324                  9320      2225 total

After:

    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     55.49    0.176638          38      4702           lstat
     24.76    0.078826          35      2225      1435 stat
     10.19    0.032434          44       733        16 open
      6.19    0.019719          36       550           fstat
      3.37    0.010723          32       336           pread
    ------ ----------- ----------- --------- --------- ----------------
    100.00    0.318340                  8546      1451 total

PR-URL: https://github.com/nodejs/node/pull/9196
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/module.js

@@ -171,8 +171,8 @@ Module._findPath = function(request, paths, isMain) {
     var basePath = path.resolve(curPath, request);
     var filename;
 
+    const rc = stat(basePath);
     if (!trailingSlash) {
-      const rc = stat(basePath);
       if (rc === 0) {  // File.
         if (preserveSymlinks && !isMain) {
           filename = path.resolve(basePath);
@@ -193,13 +193,13 @@ Module._findPath = function(request, paths, isMain) {
       }
     }
 
-    if (!filename) {
+    if (!filename && rc === 1) {  // Directory.
       if (exts === undefined)
         exts = Object.keys(Module._extensions);
       filename = tryPackage(basePath, exts, isMain);
     }
 
-    if (!filename) {
+    if (!filename && rc === 1) {  // Directory.
       // try it with each of the extensions at "index"
       if (exts === undefined)
         exts = Object.keys(Module._extensions);