package.mill support should require explicitly enablement

[lefou]

Currently, Mill is always scanning for module.sc files. We already discussed and agreed on the enablement of sub-modules through module.sc files via an $-import in PR #3213.

What could be a good name for the import? One of the candidates was submodules:

import $submodules._

[lihaoyi]

import $submodules._ works for me. Eventually we may have some other way of passing flags, but for now this will do