In reply to @nnsee "fwiw, i just did": I just did:
1) nimble init foobar (choose binary, everything else is default)
2) nimble setup
3) nimble build
4) added json_rpc to requires manually
5) added `import json_rpc` to src/foobar.nim
5) nimble build (error)
6) nimble setup
7) nimble build (error)