module: fix loading from global folders on Windows

Code was calculating $PREFIX/lib/node relative to process.execPath, but
on Windows process.execPath is $PREFIX\node.exe whereas everywhere else
process.execPath is $PREFIX/bin/node (where $PREFIX is the root of the
installed Node.js).

PR-URL: nodejs/node#9283
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/module.js

@@ -616,7 +616,16 @@ Module._initPaths = function() {
     homeDir = process.env.HOME;
   }
 
-  var paths = [path.resolve(process.execPath, '..', '..', 'lib', 'node')];
+  // $PREFIX/lib/node, where $PREFIX is the root of the Node.js installation.
+  var prefixDir;
+  // process.execPath is $PREFIX/bin/node except on Windows where it is
+  // $PREFIX\node.exe.
+  if (isWindows) {
+    prefixDir = path.resolve(process.execPath, '..');
+  } else {
+    prefixDir = path.resolve(process.execPath, '..', '..');
+  }
+  var paths = [path.resolve(prefixDir, 'lib', 'node')];
 
   if (homeDir) {
     paths.unshift(path.resolve(homeDir, '.node_libraries'));