The Library package offers the PassepartoutImplementations target for
OpenVPN/OpenSSL and WireGuard/Go, but it doesn't need it itself. Only
the main app does, so move the dependency there.
On the other side, drop the potentially problematic AppUI meta target.
Move platform filters to the Xcode project.
Indirectly fixes a crash with Xcode 16 Previews on iOS (forced to use
legacy previews before):
https://forums.developer.apple.com/forums/thread/756681
Move Core Data tests out of the Library package so that we can still use
the more efficient `swift test` for most tests.
Create a PassepartoutTests target only for tests that require
`xcodebuild`, like Core Data tests.
Eventually:
- PRs only run SwiftPM tests
- Releases run ALL tests with `scan` before `gym`
Davide