`swiftly init` installs on top of an existing swiftly at the same version without warning the user

[cmcgee1024]

If you happen to invoke swiftly init and swiftly is already installed into the current user account it will present the same prompt as a fresh install without giving a clear indication to the user what's going on.

Instead, there should be some kind of a warning indicating that swiftly is already installed and this will effectively install a new version over top of the existing one because that is what is about to happen.