Use the correct full path links for relative module references

Author: jashkenas

modules/index-all.js

@@ -1,2 +1,2 @@
-export { default } from './index-default';
-export * from './index';
+export { default } from './index-default.js';
+export * from './index.js';

modules/index-default.js

@@ -1,5 +1,5 @@
-import * as allExports from './index';
-import { mixin } from './index';
+import * as allExports from './index.js';
+import { mixin } from './index.js';
 
 // Add all of the Underscore functions to the wrapper object and return it.
 export default mixin(allExports);