MR refactor: Folder structure does not reflect store strucutre

Just tech debt, not a blocker, but before we add anything into this store, we need to refactor the folder structure.

We have a modules folder with an index file and then everything that belongs to that module is outside of that folder.

Should be:

modules
  |
  | diff
     |
     | state.js
     | getters.js
     | mutations.js
     | mutation_types.js
     | index.js
  | index.js

/cc @timzallmann @andr3