Allow -Wconf to match "unused" details

[Daenyth]

I'm trying to maintain a crossbuilding codebase, and in scala 2.13 the scala.collection.compat imports are of course marked as unused.

I tried this to silence those:

-Wconf:msg=scala.collection.compat:is
-Wconf:cat=unused&origin=scala.collection.compat:is

But neither msg nor origin seem to work to match on the error. It would be good if there would be a way to match on this

[som-snytt]

Duplicates #12326 which was just fixed in scala/scala#9939

This use case prompted the original ticket.

[SethTisue]

@som-snytt Yay! But also, I think you could improve the PR description of scala/scala#9939 to use this as an example. I've added the "release notes" label to that PR — it's more significant that I had originally realized.

[SethTisue]

@Daenyth if you want to test this in a nightly build of 2.13.9, see https://stackoverflow.com/a/40622879/86485