Move generated package to `src/` layout

[sjvrijn]

Description

This PR moves the generated package to the src/ based layout. I think I've propagated this change everywhere necessary, and all tests currently pass.

• [x] I have read the contribution guidelines
• [x] This update is in line with what is recommended in the Python chapter of the Guide

Related issues:

• fixes #173

[Cmurilochem]

Hi @sjvrijn and @egpbos . I could take care of this one and provide a review as soon as I get some time (probably next week or so). Please, could you add me as a reviewer ? Thanks!

[egpbos]

@Cmurilochem I was puzzled why you didn't have access, but for some reason you weren't part of the NLeSC/employees group (which has access to the repo). Now you are and you can do reviews (and everything else) :)