Avoid module duplication

[m-sureshraj]

If we require the same module in multiple places the bundler adds the same dependency multiple times into the bundle. Eg.

// main.js
const a = require('./a');
const b = require('./b');

// a.js
module.exports = () => {};

// b.js
const a = require('./a');

module.exports => () => { a(); };

The bundler will generate following dependency graph

main module <- a module <- b module <- a module

We can avoid this duplication by checking module existence in the graph. Inside the getModules function

module.requires.forEach(dependency => {
  const basedir = path.dirname(module.filepath)
  const dependencyPath = resolve(dependency, { basedir })

  // to avoid module duplication
  let dependencyObject = modules.find(m => m.filepath === dependencyPath);

  if (dependencyObject) {
    module.map[dependency] = dependencyObject.id
   } else {
    dependencyObject = createModuleObject(dependencyPath)
    module.map[dependency] = dependencyObject.id
    modules.push(dependencyObject)
  }
})

@adamisntdead I would like create a PR for the above changes, If you like this solution. Thanks.