Add supports for importing a folder with trailing slash on the path

[jihchi]

Currently, we will always append /index.js to the folder path when we import a directory index.js file, for example:

import components from './Foo';
// becomes
import components from './Foo/index.js';

It will be problematic when we serve the files and load it in browser, for example, say we have following folder structure:

1. ./src/index.js

import Foo from './Foo/'; // notice that there is a trailing slash
console.log(Foo);

2. ./src/Foo/index.js

import Bar from '../Bar.js'; // notice that this is a relative import
export default "Foo " + Bar;

3. ./src/Bar.js

export default "Bar";

now we serve the src folder on http://localhost:8080 and load http://localhost:8080/index.js in browser:

1. in the ./src/index.js file, syntax import Foo from './Foo/'; becomes import Foo from './Foo//index.js'; and then
2. browser will fetch the file by the URL http://localhost:8080/Foo//index.js, this will work, but then
3. browser will fetch ../Bar.js by the URL http://localhost:8080/Foo/Bar.js, this won't work, we will get 404 error.

---

This PR attempts to fix the issue by simply check whether the path does ends with trailing slash.

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[FredKSchott]

+1, makes sense to me. Thanks for contributing!