[Vertex AI] Enable `AccessLevelOnImport` and `InternalImportsByDefault`

[andrewheard]

WIP - Blocked on #14627 and #14638

Enabled the experimental (in Swift 5.x) feature AccessLevelOnImport and the upcoming opt-in feature InternalImportsByDefault for the Vertex AI SDK. AccessLevelOnImport will be the default in Swift 6.x and can be removed once #14627 and #14638 have been completed.

This PR starts resolving #13031 by removing uses of @_implementationOnly in the FirebaseVertexAI target.

#no-changelog

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[paulb777]

Is this PR still relevant?

[ncooke3]

Yes, but the Package.swift changes can wait for Firebase 12. And internal could explicitly be added to imports for now.

[andrewheard]

Yes, but the Package.swift changes can wait for Firebase 12. And internal could explicitly be added to imports for now.

@ncooke3 @paulb777 If I can do it quickly, I'd like to see if I can continue to use InternalImportsByDefault (removed AccessLevelOnImport though). I'd be more comfortable that I haven't missed something and unintentionally made it public.

[ncooke3]

Close in favor of #15094