E3-986: Remove BuildResults object

This removes the BuildResults object, in favour of having the Builder return lists of successful/failed builds. Note that as we are still in "fail fast" mode, there is a bit of silliness involved in this (we never use the failed builds). However, I think it is still worth having them in there in order to make it abundantly clear what to change if we want to switch from "fail fast" to "try to build as much as possible".

Note that the WrapperRegistry has also lost the use_ssh parameter; instead, it keeps both the ssh/https urls which can then be fetched by the builder when cloning.