Switch over to Error Prone (now with NullAway!)

There is one major change: we now have a separate artifact for the annotation processor.

As for NullAway, we are currently exempting the clientbound join game/respawn packets. They are ugly and need to be refactored.

api/build.gradle

@@ -7,6 +7,7 @@ plugins {
 apply plugin: 'org.cadixdev.licenser'
 apply from: '../gradle/checkstyle.gradle'
 apply from: '../gradle/publish.gradle'
+apply from: '../gradle/errorprone.gradle'
 apply plugin: 'com.github.johnrengelman.shadow'
 
 java {
@@ -18,12 +19,6 @@ license {
     header = project.file('HEADER.txt')
 }
 
-sourceSets {
-    ap {
-        compileClasspath += main.compileClasspath + main.output
-    }
-}
-
 dependencies {
     api 'com.google.code.gson:gson:2.8.6'
     api "com.google.guava:guava:${guavaVersion}"
@@ -55,20 +50,10 @@ task javadocJar(type: Jar) {
 task sourcesJar(type: Jar) {
     classifier 'sources'
     from sourceSets.main.allSource
-    from sourceSets.ap.output
-}
-
-jar {
-    from sourceSets.ap.output
-}
-
-shadowJar {
-    from sourceSets.ap.output
 }
 
 artifacts {
     archives javadocJar
-    archives shadowJar
     archives sourcesJar
 }