[K/N][build] Composite build with build.tools-conventions plugin

Adds convention plugin instead of sources inclusion
inside the buildSrc, effectively reducing the amount
of the code to be compiled in buildSrc.

Merge-request: KT-MR-8898

build.gradle.kts

@@ -39,6 +39,9 @@ plugins {
     signing
     id("org.jetbrains.kotlin.jvm") apply false
     id("org.jetbrains.kotlin.plugin.serialization") apply false
+    if (kotlinBuildProperties.isKotlinNativeEnabled) {
+        id("kotlin.native.build-tools-conventions") apply false
+    }
 }
 
 pill {