Split packages into separate modules

There are a number of reasons why the current structure of this repository is not the best:

- Even if I want to use one package from this module, Go will install everything listed in `go.mod`
- gqlgen@v0.11.0 broke backwards compatibility with this module. If I update the lowest required version in `go.mod` to v0.11.0 to fix the code in this module, I will have to do that for all components, otherwise the others would stop working.

Therefore I suggest splitting this repository into smaller modules. I'd be happy to upgrade the opencensus integration to v0.11.0