checkUnusedDependencies could improve compileOnly dependencies

[schlosna]

What happened?

I've been testing out checkUnusedDependencies on tritium (see palantir/tritium#200) and encounter the following error for a compileOnly dependency on auto-service

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':tritium-registry:checkUnusedDependencies'.
> Found 1 dependencies unused during compilation, please delete them from 'tritium-registry/build.gradle' or choose one of the suggested fixes:
        com.google.auto.service:auto-service

Currently the dependencies are defined as:

    annotationProcessor 'com.google.auto.service:auto-service'
    compileOnly 'com.google.auto.service:auto-service'

If I remove the compileOnly 'com.google.auto.service:auto-service' from tritium-registry/build.gradle then compilation fails as DefaultTaggedMetricRegistry because we import com.google.auto.service.AutoService.

Compiling with JDK Java compiler API.
/Volumes/git/tritium/tritium-registry/src/main/java/com/palantir/tritium/metrics/registry/DefaultTaggedMetricRegistry.java:20: error: package com.google.auto.service does not exist
import com.google.auto.service.AutoService;
                              ^
/Volumes/git/tritium/tritium-registry/src/main/java/com/palantir/tritium/metrics/registry/DefaultTaggedMetricRegistry.java:22: error: cannot find symbol
@AutoService(TaggedMetricRegistry.class)
 ^
  symbol: class AutoService
2 errors

> Task :tritium-registry:compileJava FAILED

What did you want to happen?

A clear mechanism to properly handle compileOnly dependencies with checkUnusedDependencies