Backport "Support src filter in -WConf (Closes #17635) (#18783)"

[mkurz]

Backports

• Support src filter in -WConf (Closes #17635) #18783 (commit: 86e2fe5)

for next 3.3.4 release.

Besides import conflicts cherry-picking was smooth.

[mkurz]

Any chance someone looks at this PR or assign the 3.3.4 milestone? Thanks!

[SethTisue]

Just chiming in to say that I think this would be highly beneficial to see merged for 3.3.4. Beneficial in general, but also specifically for our needs at Lightbend. (And of course @mkurz represents Play, obviously important users of Scala.)

There doesn't seem to be a 3.3.4 milestone I can assign. I'll take the liberty — which I try not to take too often! — of mentioning @Gedochao and @Kordyjan because I'd like to understand what the intended workflow is here. Like, is the target branch being release-3.3.4 sufficient for the PR to be noticed when 3.3.4 is put together? Or is there some label I could apply in a case like this? I thought about "backport:nominated" but it doesn't seem exactly apropos since this PR actually is a backport 😄