Use an internal Swift package

[florentmorin]

Without changing lots of code, performances (runtime, compilation), security and quality can be improved with an internal Swift package.
It can also helps to easily share code with potential extensions.

[nathanfallet]

Not sure it will be useful. If we need to share code with extensions, we can include a file in multiple targets directly.

[PlugNPush]

I think that for the usage we currently have, which is an expanding, just launched app, an Internal Swift Package would potentially even be worse to work in an efficient manner.