"sources" option for libraries

[TheOnlySilverClaw]

As far as I understand, files in libraries currently have to be on the root folder next to the manifest.json.

For bigger libraries, it would be helpful to set a "sources" path, like in projects, for organizing directory structures.

[lerno]

That's something worth adding.

[tonis2]

I think it can be related to this issue too #1627

[lerno]

This should be working now.