Add export specification

[tobiasdiez]

Define Package entry points using the exports field in package.json. There might be additional paths that you want to expose and I'm not sure if the utilities should be exposed by this package.json file or by its own one.

Fixes #9048 and probably #9938, #9156, #8218, apollographql/apollo-feature-requests#287

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests