Provide an alternative to `import $file`

[lihaoyi]

This syntax is an awkward holdover from Ammonite. Ideally we would make every .mill file in the same package live in the same namespace, but I couldn't figure out how to do that in Mill0.12/Scala2. Maybe in Mill0.13/Scala3 we have a few more options

[lefou]

So we replaced the explicit inclusion of files with the implicit inclusion of all .mill files ins the same directory. Personally, I don't like that so much, since now Mill picks up files in an uncontrolled manner. I can understand, that some users want this feature, but I think it would be better to follow our long tradition in enabling it explicitly.