[FR]: Switch from `@_implementationOnly` to Swift 5.9 internal package imports

[paulb777]

Description

Investigate switching from @_implementationOnly to Swift 5.9 internal package imports

API Proposal

No response

Firebase Product(s)

All

[andrewheard]

The AccessLevelOnImport feature being introduced in Swift 6 (SE-0409) works in Swift 5.9 by setting the following on a target in Package.swift:

swiftSettings: [
  .enableExperimentalFeature("AccessLevelOnImport")
]

However, in targets with resources, e.g., resources: [.process("Resources/PrivacyInfo.xcprivacy")], the resource_bundle_accessor.swift generated by SPM does not specify access levels resulting in errors like Ambiguous implicit access level for import of 'Foundation'; it is imported as 'public' elsewhere.

The upcoming feature InternalImportsByDefault doesn't appear to be available in Xcode 15.x but the issues above are resolved in Xcode 16. Let's hold off on using the new access-level modifiers on import declarations until Firebase moves to a minimum of Xcode 16 in the future.

[paulb777]

Mostly done and the rest is covered by #14616