Unify MillBuildModule with MillBuildRootModule

[lihaoyi]

MillBuildModule is basically a lightweight partial implementation of MillBuildRootModule. Now that we have MillBuildRootModule, we should use it

[lefou]

Also, expose the mill-build to be used by mill. Maybe we can expose it as an external module. Or we could invent some cli arg to enable the meta-build.