Disallow naming the root package, except for selections

[dwijnand]

References #18020, didn't actually fix that, because it addressed aspects of using _root_ in definitions. Adding more details to that ticket.
Fixes #17757
Fixes #18050
Includes the code in scala/bug#12508

[som-snytt]

Just to note that Scala 3 is so great with soft modifiers, that I don't see why _root_ isn't handled as in Scala 2, where _root_ is special only in a selection, and you also get a warning if _root_ does not mean _root_-ed path. That avoids both making it a keyword and accidental self-injury by users.

[dwijnand]

Because I feel like it's entirely unnecessary - I think Scala 2 is trying too hard for no real reason.