Don't search the same plugin path twice

Fix part of #379

lib/plugin.js

@@ -137,11 +137,18 @@ Plugin.installed = function() {
 
   var plugins = [];
   var pluginsByName = {}; // don't add duplicate plugins
+  var searchedPaths = {}; // don't search the same paths twice
   
   // search for plugins among all known paths, in order
   for (var index in Plugin.paths) {
     var requirePath = Plugin.paths[index];
     
+    // did we already search this path?
+    if (searchedPaths[requirePath])
+      continue;
+      
+    searchedPaths[requirePath] = true;
+    
     // just because this path is in require.main.paths doesn't mean it necessarily exists!
     if (!fs.existsSync(requirePath))
       continue;