Access modifiers on imports in generated files

### Motivation

Starting in Swift 6.0, users can set `InternalImportsByDefault` as an upcoming feature flag, which makes imports internal by default. Then, if they add `accessModifier: package/public`, the code fails to build.

### Proposed solution

There should be some way to get the access modifier added on the imports as well.

One way: put `if #compiler(>=6.0)` around imports with the access modifier, and `#else` won't have it. That way we can do it without other feature flags.

### Alternatives considered

_No response_

### Additional information

_No response_

Locate the generator entry point and the code responsible for rendering imports in generated Swift files. Check how the accessModifier option is propagated, then run the relevant generator tests or fixtures to verify package/public imports compile under Swift 6 while older compiler behavior remains unchanged.

Written by the indexing model from the issue text.