Build: Replace buildSrc with a separate included build (#3104)

Gradle doesn't allow us to substitute dependencies of buildSrc
with artifacts provided by a composite build. But our buildSrc
depends on kotlin-native-utils provided by the Big Kotlin repo.
Thus we cannot make changes in kotlin-native-utils and use them
in buildSrc during a development process.

This patch fixes the issue by transforming buildSrc into a separate
included build.

gradle.properties

@@ -1,5 +1,5 @@
 #
-# Copyright 2010-2017 JetBrains s.r.o.
+# Copyright 2010-2019 JetBrains s.r.o.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -26,8 +26,3 @@ testKotlinCompilerVersion=1.3.50-dev-1455
 konanVersion=1.3.50
 org.gradle.jvmargs='-Dfile.encoding=UTF-8'
 org.gradle.workers.max=4
-#kotlinProjectPath=/Users/jetbrains/IdeaProjects/kotlin
-#sharedProjectPath=/Users/jetbrains/IdeaProjects/kotlin-native-shared
-#kotlinProjectPath=/Users/jetbrains/kotlin-native/kotlin
-#sharedProjectPath=/Users/jetbrains/kotlin-native/kotlin-native-shared/kotlin-native-shared
-