Dependencies still have 4 possible fields. 

It used to be `(name, version, versionsuffix, dummy)`, with the last one a `bool` indicating whether a dummy toolchain was used or not. Now it is `(name, version, versionsuffix, toolchain)`, with the last one still accepting `bool` for indicating `dummy`, while `list` or `tuple` value indicates a custom toolchain spec. So, nothing broke...