Allow package dirs

[nmatei]

allowing packageDirs to be able to use our own folders for packages & more important to be able to use useWorkspaces: true in lerna.json

[nmatei]

I've seen that "@sencha/ext-webpack-plugin" took place for "@sencha/ext-react-webpack-plugin", but that is not in public repo anymore...
or entire plugin will be deprecated?

[jnsencha]

@nmatei I've reviewed your work. Unfortunately we don't use this repository any longer, it will be removed. You are welcome to submit a PR for the webpack plugin updates to the ext-webpack-plugin package located here: https://github.com/sencha/ext-allshared/tree/ext-allshared-7.4.x/packages/ext-webpack-plugin. You can also submit a PR against this repo for the artifacts file: https://github.com/sencha/ext-allshared/blob/e89272add4019d73ba5955786a1438c1a11b3a39/packages/ext-webpack-plugin/src/artifacts.js#L196

Thanks!

[jnsencha]

@wemersonjanuario could you review these changes as well please?