Remove mill._ magic import from builds

[lolgab]

Follow up to discussion in #2381

[lolgab]

MillBuildRootModule was also imported via a magic import.
Should we require users to import mill.runner._ or should we add a type alias in the package object mill?

[lihaoyi]

Requiring a mill.runner._ import is probably not what we want, from a user perspective

The reason we need a separate import is because the mill._ import is in main, but mill.runner.MillBuildRootModule is in runner

I think we don't need to fix that right now. It's definitely a bit awkward, but it's not causing any user-facing issues, so we can take our time and fix it next binary incompatible release