This allows using a project reference that points to a readonly
directory.
The use case for this is having sbt plugin projects on a network
share (readonly) that you can just point to. The plugin projects
get copied and built automatically, just like a git project
reference gets cloned and built.
This will ease plugin imcompatibilies between minor sbt versions,
avoiding to have to cross build plugins against all compatible sbt
versions.
Fred Dubois
4ffe240eca