`headerVisibility` should default to `project` for dynamic framework targets

[armcknight]

It currently defaults to public for any header, but this does not match Xcode's behavior, which is to assign any newly created .h file to project visibility in a dynamic framework's Headers Build Phase:

This could easily allow an accidental exposure of API not meant for public consumption, which is a bad spot for an SDK to be in, as once that version is released, if adhering to semver, fixing that visibility would require a major version bump.