Allow rebuilding without changing the package

[antoniusnaumann]

Rebuilding the Swift package every time is not practical for use cases where custom Swift code or tests on Swift side should be added.

There are multiple solutions to this:

1. Keep cargo swift in control of the whole package for every build and let the user include the generated package in a separate package that handles Swift tests and code.
2. Use Swift Package only to generate the package. Introduce a cargo swift build command that only rebuilds the binaries and bridges.
3. Let the package command decide itself if a new package should be initialized or an existing one should be updated.