The generated XML / XJC code pretty much never changes but because
everything is lumped together in one compile task, we had to recompile
this every time anything else in core/ changes. It's technically
possible to use a separate source set for the XJC code while having it
remain in core/, but splitting it into a separate subproject gives
better separation and makes the caching / dependency graph clearer.
Pretty much all, except for the caching configuration, of the new
xjc/build.gradle is taken from the previous core/build.gradle.
