Switch the core library to be non-null by default

See comments in c8c128d335 for further
details. This requires /relatively/ few changes - mostly cases we were
missing @Nullable annotations.

buildSrc/src/main/kotlin/cc-tweaked.java-convention.gradle.kts

@@ -6,6 +6,7 @@ import java.nio.charset.StandardCharsets
 
 plugins {
     `java-library`
+    idea
     jacoco
     checkstyle
     id("com.diffplug.spotless")
@@ -139,3 +140,12 @@ spotless {
         ktlint().editorConfigOverride(ktlintConfig)
     }
 }
+
+idea.module {
+    excludeDirs.addAll(project.files("run", "out", "logs").files)
+
+    // Force Gradle to write to inherit the output directory from the parent, instead of writing to out/xxx/classes.
+    // This is required for Loom, and we patch Forge's run configurations to work there.
+    // TODO: Submit a patch to Forge to support ProjectRootManager.
+    inheritOutputDirs = true
+}