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Issue Description:

See the attached sample project and run `swift build` it should fail but it doesn't, see below for details.

Currently there is a case with transitive modules where you do not have to import a module to get extensions from it if you import the module containing the type its extending, and a module that depends on the module adding the extension. This is fixed if you add `@_implementationOnly` to the import of the module that adds the extension, but ideally it would be required to import any module you use an extension from (unless it's explicitly `@_exported`

In the sample project you can see that `import transitive` pollutes the `funcFromExtension` into the main `import-pollution` target. By changing the import to `@_implementationOnly import extender` you fix this issue.

I know this has been an issue for a long time since this also applies to extensions on types defined in system modules, and because of that it may never be "fixed" since it would be a source incompatible change, but I wanted to have an issue about this since I couldn't find any others.

In projects with many modules this makes it hard to keep your dependency graph actually explicit and introduces surprises when you remove a dependency on a module and end up losing transitive dependencies you were implicitly depending on.