[Sema] Integrate @_spiOnly imports logic with recent API checks

[xymus]

We recently added checks to prevent SDK breakages and downgraded to warnings until Swift 6. Since @_spiOnly is a new feature we can skip the warning step and apply them as errors right away.

1. Public use of a typealiases that desugars to something from an @_spiOnly import is reported as an error.
2. Public use of an @_spiOnly imported conformance is reported as an error.
3. Accept @_spiOnly imports of a private module from a public module.

This follows the exportability check added in #61011. Next I’ll add reports of conflicting imports from the same file, and look into better integration with the SPI library-level.

[xymus]

@swift-ci Please smoke test