[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

kotlin-native/libllvmext/build.gradle.kts

@@ -14,14 +14,15 @@
  * limitations under the License.
  */
 
-import org.jetbrains.gradle.plugins.tools.lib
+import org.jetbrains.kotlin.tools.lib
 import org.jetbrains.kotlin.*
 import org.jetbrains.kotlin.konan.target.ClangArgs
 import org.jetbrains.kotlin.konan.target.Family.*
 import org.jetbrains.kotlin.konan.target.HostManager
 
 plugins {
-    `native`
+    id("kotlin.native.build-tools-conventions")
+    id("native")
 }
 
 native {