Support monorepos

[rvcas]

We need to support monorepos so that people can have projects that contain both offchain and onchain code seamlessly.

This is also useful because perhaps someone wants to have many different aiken libraries within one git repository.

This is also kind of industry standard at this point and cargo from Rust is really good prior art.

We should ideally just copy cargo but only bring over the minimum amount we need for our goals. There is no need to have complete parity with cargo in regards to config files and monorepo setup.