Add SystemOut checker

[tbroyer]

This checker checks for uses of System.out and System.err as
possible logs used for debugging purpose.

Inspired by Policeman's Forbidden API Checker's jdk-system-out checks.

See also #1211

[cushon]

Is the idea for this to be disabled by default, but make it available so projects where using System.{err,out} is never desirable (servers, libraries, etc.) can opt in to it? That seems like a good approach. I've seen related bugs in code that's deliberately using System.out (e.g. CLIs) where print statements from debugging accidentally get committed to the production version. But that seems like a harder problem to find good heuristics for.

[tbroyer]

I would personally always enable it, and @SuppressWarnings("SystemOut") where it makes sense.
One heuristic that would be useful (to me) would be similar to SystemExitOutsideMain: allowing System.{out,err} in classes where a main() method is present. That wouldn't work for most CLIs though.

The reason I put it in the disabled-by-default list was mostly that SystemExitOutsideMain is there too.

[JakeWharton]

I DI System.out and System.err from main but otherwise pass around Print* classes or use a proper logging framework.

[cushon]

Thanks for the reminder about SystemExitOutsideMain, I'm not sure if something similar had been discussed for System.out or if I was just half-remembering that check. The heuristic for System.out seems trickier.

I think that requiring @SuppressWarnings("SystemOut") or DI'ing System.{out,err} are both sensible policies for a codebase to adopt, but might be too opinionated to have in the default EP configuration. We've generally been trying to make the defaults reflect things that are probably bugs, not just things that are required e.g. by Google best practices. (We don't always achieve that, but help staying honest is appreciated.)

I think this makes sense to make available as an opt-in / disabled-by-default check.

[tbroyer]

I rebased the branch, in case it can unblock copybara.