Warn/error if an entrypoint and the same field are defined in the same file

[rbalicki2]

This causes TS issues

We should at least put up some docs about this

Perhaps the compiler should error if there is an entrypoint and a field that is accessible from that entrypoint (perhaps only top level? perhaps only directly? maybe its fine if its loadable) in that same file

[rbalicki2]

related to #151