See my minimal reproduction here:
https://github.com/Zamiell/tstl-monorepo

Related to #694, but I think it might be different because I am specifically specifying one package.

I think that this feature is necessary to have TypeScriptToLua projects in a monorepo. The typical use case for a monorepo is that you put a client and a server together in the same repo, and then have a "common" package that contains the shared types that the client and the server import in turn.