Improve UX when using default tasks

[lefou]

Default tasks allow shorter CLI arguments. E.g. mill foo.test.testForked can be written as mill foo.test. While convenient, it hides the effective structure of the project. Therefore I propose to output some info when Mill applies a defaultTask to make a CLI call work. Users not so familiar with the project structure have a chance to learn what really happens en passant.

> mill foo.test
Expanding default task of foo.test => foo.test.testForked

Originally posted by @lefou in #5898

[lihaoyi]

I think there isnt anything we can do here better than the status quo. Printing out such a message every time someone runs tests would be spammy and useless 99% of the time, and devalues all other logs we print. And we already print the full task name in the prompt, above any logs the test has, and again in the event of failure. And the default task of testForked is documented on the docsite

We could do things, but I don't see anything we could do that would make things better overall

[lefou]

Ok, maybe we add that info only when --debug option is used.