Scaladoc: better `-external-mappings` (working on packages instead of paths)

[Florian3k]

Scaladoc can link to documentation of external types/classes/etc on other websites via -external-mappings option (https://docs.scala-lang.org/scala3/guides/scaladoc/settings.html#-external-mappings).

This option works by matching the path to a file, not package, which is very error prone (VirtusLab/scala-cli#2751)

Instead, it would be better to make it work on packages