Fix Foundation import's access level in generated code

[gjcairo]

#1683 added support for access level modifiers on imports in the generated code.

Foundation (which is needed because Data is used extensively in generated protos) is imported by manually writing to the file instead of using the "compute imports" logic we use to deal with all other imports. This is because other imports are all imports coming from other generated proto files, whereas Foundation is "pure Swift" dependency.

We forgot to add the access level modifier to this Foundation import.
This causes issues when enabling the InternalImportsByDefault flag on modules that use generated protos (even when swift-protobuf's UseAccessLevelOnImports option is enabled), because Foundation will be imported as internal, while we expose public properties of type Data in the generated code.

This PR adds the right import Foundation statement based on the import directive.

[thomasvl]

@gjcairo can you also test this option with a public import file? Glancing back the way we re-export things it seems like that might not be right, as won't it put public on things where things would otherwise be package or internal and is that the correct net result?

[gjcairo]

@thomasvl according to the proposal, @_exported imports must be followed by public if using access level modifiers:

With this proposal, @_exported is accepted only on public import declarations, both with the modifier or the default public visibility in current language modes.

So I'd say this is the correct behaviour.