Consolidate import $file.foo.bar into build.foo.bar namespace

[lihaoyi]

This would make it more consistent with how Scala source files behave in general and allow IDEs to understand the references without any customization. The old syntax can remain for migration compatibility

Might need to wait for 0.13.0 depending on whether this needs scala 3 or whether it needs to break bincompat (not sure). We should at the same time split Discover[T] into an API trait and implementation class to minimized the surface area constrained by binary compatibility requirements