Check if module path is already loaded before doing fs operations

resolve now checks the existence of a module already loaded at the
given path before checking if the file exists on disk.

This removes the need for many redundant fs.exists and fs.isFile
calls for already loaded modules.

This speeds up package loading since most packages have a common
set of requires that were doing needless fs operations for each
searched path when the module was already in the cache at that
path.

src/stdlib/require.coffee

@@ -73,16 +73,25 @@ resolve = (name, {verifyExistence}={}) ->
     file = file.replace '../', "#{prefix}/"
 
   if file[0] isnt '/'
-    paths.some (path) ->
-      fileExists = /\.(.+)$/.test(file) and __exists "#{path}/#{file}"
-      jsFileExists = not /\.(.+)$/.test(file) and __exists "#{path}/#{file}.js"
-
-      if jsFileExists
-        file = "#{path}/#{file}.js"
-      else if fileExists
+    moduleAlreadyLoaded = paths.some (path) ->
+      if __moduleExists "#{path}/#{file}"
         file = "#{path}/#{file}"
-      else if expanded = __expand "#{path}/#{file}"
+      else if __moduleExists "#{path}/#{file}.js"
+        file = "#{path}/#{file}.js"
+      else if expanded = __moduleExpand "#{path}/#{file}"
         file = expanded
+
+    if not moduleAlreadyLoaded
+      paths.some (path) ->
+        fileExists = /\.(.+)$/.test(file) and __exists "#{path}/#{file}"
+        jsFileExists = not /\.(.+)$/.test(file) and __exists "#{path}/#{file}.js"
+
+        if jsFileExists
+          file = "#{path}/#{file}.js"
+        else if fileExists
+          file = "#{path}/#{file}"
+        else if expanded = __expand "#{path}/#{file}"
+          file = expanded
   else
     file = __expand(file) or file
 
@@ -92,16 +101,27 @@ resolve = (name, {verifyExistence}={}) ->
     console.warn("Failed to resolve '#{name}'") if verifyExistence
     null
 
+__moduleExists = (path) ->
+  __modules[path] isnt undefined
+
+__moduleExpand = (path) ->
+  return path if __moduleExists path
+  for ext, handler of exts
+    return "#{path}.#{ext}" if __moduleExists "#{path}.#{ext}"
+    return "#{path}/index.#{ext}" if __moduleExists "#{path}/index.#{ext}"
+  null
+
 __expand = (path) ->
+  modulePath = __moduleExpand path
+  return modulePath if modulePath
+
   return path if __isFile path
   for ext, handler of exts
-    if __exists "#{path}.#{ext}"
-      return "#{path}.#{ext}"
-    else if __exists "#{path}/index.#{ext}"
-      return "#{path}/index.#{ext}"
+    return "#{path}.#{ext}" if __exists "#{path}.#{ext}"
+    return "#{path}/index.#{ext}" if __exists "#{path}/index.#{ext}"
 
   return path if __exists path
-  return null
+  null
 
 __exists = (path) ->
   $native.exists path