suggested improvements to `com.diffplug.eclipse.apt`

The `com.diffplug.eclipse.apt` plugin is just copy-pasted with a few minor fixes from the https://github.com/tbroyer/gradle-apt-plugin, which is no longer maintained. It is no longer maintained because Gradle incorporated native support for annotation processing, so the plugin became extraneous. The one exception is gradle+eclipse, where annotation processing still requires this plugin.

Per advice from the original author, it would be good to remove the plugin DSL and extract annotation processor arguments from [getAllCompilerArgs](https://docs.gradle.org/current/javadoc/org/gradle/api/tasks/compile/CompileOptions.html#getAllCompilerArgs--) instead; and remove the AptSourceSetConvention, remove/inline AptPlugin52, and remove the AptPlugin altogether.

When/if `eclipse.apt` generates problems or deprecation warnings in a future release, I'll consider taking this on if it's the easiest way to fix them. So long as the current code works perfectly, it will not make the top of my todo list. If it makes the top of anyone else's todo list, I'll be happy to merge these improvements :)