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.

generators/build.gradle.kts

@@ -4,6 +4,16 @@ plugins {
     id("jps-compatible")
 }
 
+sourceSets {
+    "main" { }
+    "test" { projectDefault() }
+}
+
+val builtinsSourceSet = the<JavaPluginConvention>().sourceSets.create("builtins") {
+    java.srcDir("builtins")
+}
+val builtinsCompile by configurations
+
 dependencies {
     compile(projectTests(":compiler:cli"))
     compile(projectTests(":idea:idea-maven"))
@@ -18,17 +28,15 @@ dependencies {
     compile(projectTests(":kotlin-noarg-compiler-plugin"))
     compile(projectTests(":kotlin-sam-with-receiver-compiler-plugin"))
     compile(projectTests(":generators:test-generator"))
+    builtinsCompile("org.jetbrains.kotlin:kotlin-stdlib:$bootstrapKotlinVersion")
     testCompileOnly(intellijDep("jps-build-test"))
     testCompileOnly(project(":kotlin-reflect-api"))
     testCompile(intellijDep("jps-build-test"))
+    testCompile(builtinsSourceSet.output)
     testRuntime(intellijDep()) { includeJars("idea_rt") }
     testRuntime(projectDist(":kotlin-reflect"))
 }
 
-sourceSets {
-    "main" { }
-    "test" { projectDefault() }
-}
 
 projectTest {
     workingDir = rootDir
@@ -41,4 +49,6 @@ val generateProtoBufCompare by generator("org.jetbrains.kotlin.generators.protob
 
 val generateGradleOptions by generator("org.jetbrains.kotlin.generators.arguments.GenerateGradleOptionsKt")
 
+val generateBuiltins by generator("org.jetbrains.kotlin.generators.builtins.generateBuiltIns.GenerateBuiltInsKt", builtinsSourceSet)
+
 testsJar()