Re-merge all build definitions into one file

[pieter-bos]

One benefit we had from splitting up the build definition is that when the build definition changes, only the targets defined in that file are invalidated (so e.g. large downloads are not repeated, because they're defined in a different file). This benefit is no longer relevant: com-lihaoyi/mill#2417

There is something to be said for keeping the build definitions nice and modular, but it is also very annoying that intellij does not recognize import $file declarations, so I am leaning towards re-merging the build definitions.