Wconf: support src filter from Scala 2

[kubukoz]

In #12857, the -Wconf options were added. However, the src filter, which some people are using to ignore warnings from generated code, has seemingly been left out.

I'm getting this error when trying to use it:

[error] [core3] Failed to parse `-Wconf` configuration: src=src_managed/.*:silent
[error] [core3] unknown filter: src

In my eyes it's more of a bug report than a feature request (since the feature exists in the 2.x versions of the language, and its lack of existence is a potential migration blocker), but I'm reporting here as a first step.

[lrytz]

I'm happy to add it if noone objects.

[pribor]

Any progress on this request?