Move builtins generators to their own source set

Make them depend on bootstrap stdlib.
This is to prevent recompiling the entire project when we need just to
generate builtins sources.

buildSrc/src/main/kotlin/CommonUtil.kt

@@ -7,6 +7,7 @@ import org.gradle.api.file.CopySourceSpec
 import org.gradle.api.file.SourceDirectorySet
 import org.gradle.api.plugins.JavaPluginConvention
 import org.gradle.api.tasks.JavaExec
+import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.SourceSetOutput
 import org.gradle.kotlin.dsl.creating
 import org.gradle.kotlin.dsl.extra
@@ -52,8 +53,8 @@ var Project.javaHome: String?
     get() = extra.takeIf { it.has("javaHome") }?.get("javaHome") as? String
     set(v) { extra["javaHome"] = v }
 
-fun Project.generator(fqName: String) = smartJavaExec {
-    classpath = the<JavaPluginConvention>().sourceSets["test"].runtimeClasspath
+fun Project.generator(fqName: String, sourceSet: SourceSet? = null) = smartJavaExec {
+    classpath = (sourceSet ?: the<JavaPluginConvention>().sourceSets["test"]).runtimeClasspath
     main = fqName
     workingDir = rootDir
 }