Introduce clients package

[c-w]

This pull request improves the organization of our package structure by introducing a wrapper package for all clients. After merging, our folder structure will be much simpler: schemas and resolvers for GraphQL plus clients for third-party integrations.

While looking at all of our require statements, I also fixed a few inconsistencies like always using const and never relying on an implicit Promise object.

[Smarker]

LGTM