publishLocal should be cached (publishLocalCached)

[lefou]

Since publishLocal is local, it can be cached. With PathRef.revalidate Mill can detect, whether a cached target pointing to files outside of it's cache region is still valid. For that to work, all published files need to be returned as PathRef with enabled revalidation flag.

This would be a binary incompatible change, hence a intermediate API like publishLocalCached could be added.

[lefou]

The main use case which will be enabled with this feature, would be the local provision of some dependencies, which are consumed via coursier, but are not remotely available. This is often the case when OSGi bundles of thirdparty dependencies are needed but not available. Without this feature, a dedicated provision step needs to be run now and then. But adding it the the dependencies is too costly, due to the fact that publishLocal is a command, which always runs. Since a cached version will not need to run if the files are already in place, those provision steps can be added to the permanent dependencies of their consumers, and there is no need to run the extra manual tasks.