Investigate SCP-009 for warnings on over-broad deps.

[johnynek]

see: scala/scala#5724

The idea is to add a flag to scalac that warns if a jar is on the classpath that is not used during compilation. With this, we could potentially make that a fatal error when given a certain flag (or of course using fatal warnings).

[softprops]

Very cool!

[ittaiz]

@johnynek should we close this in favor of #235 ?
It's more generic but it's also more indicative on the direction we're going (i.e. not to use the stock plugin but to use our own variant)

[johnynek]

yes, I think so.