Upstream `dependency:` syntax from sbt-scalafix to scalafix

[bjaglin]

Even though docs do mention that the scalafix dependency:RULE@GROUP::ARTIFACT:VERSION syntax is only usable within sbt, it's easy to miss that. See scala-steward-org/scala-steward#3486 (comment) for example.

As more build tools support scalafix (most recently VirtusLab/scala-cli#2968) and sbt no longer is the canonical way to run scalafix, it's a shame that this logic must be reimplemented.

Upstreaming the feature as-is is not trivial as support for fetching and loading external rules is currently handled in scalafix-interfaces, so scalafix-cli (where rule parsing happens) would have to re-run with a wider classpath somehow.

[bjaglin]

Somewhat related to #1625 implementation-wise