Commands should be given a dest directory

[lihaoyi]

Currently T.ctx().dest only works for labelled Targets, which are publicly accessible. Tasks (which don't have a label) and overriden Targets are not given dest directories, because it's not clear what path that dest directory would have.

Currently, Commands do not have a dest directory. Given that the Commands do have a unique path by which they are run, they too should have a dest path assigned to them for them to use for scratch & output files. This would let us avoid creating temp dirs as we do here https://github.com/lihaoyi/mill/blob/bb61c05217671e80ba381b2cac869130c306baa8/scalaplugin/src/main/scala/mill/scalaplugin/ScalaModule.scala#L20

[lihaoyi]

@prassee have you had time to take a look at this?

[lihaoyi]

282d9a6