Allow permissions override of TreeArtifact `pkg_files` root directories (using `pkg_mkdirs`?)

In some cases, we want to provide direct ownership and/or override the permissions of a TreeArtifact expanded in a package. While it is likely already possible to change permissions on directories inside TreeArtifacts using `pkg_mkdirs`, you can't change those on the root directory, as the packaging rules will complain about collisions.

While additional TreeArtifact magic could be supported in `pkg_files`, it seems that allowing `pkg_mkdirs` to collide with TreeArtifacts should be a valid choice.

This is at least a problem with `pkg_rpm`'s internal `pkg_filegroup` framework handler. It is likely present in the "common" one too.