[KGP] Fix missing task dependency between Test and Main compilations

For historic reasons friendPaths might contain the jars built from the
main module. However it's an Internal field, so task dependency wasn't
created.

This commit completes the chain of providers. As a result Gradle can
deduce that :compileKotlinTest depends on :desktopJar, if applicable.

^KT-69330 Fixed

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/tasks/AbstractKotlinCompile.kt

@@ -104,12 +104,16 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments> @Inject constr
 
     // This allows us to treat friendPaths as Input rather than InputFiles
     @get:Input
-    internal val friendPathsSet: Set<String>
+    internal val friendPathsSet: Provider<Set<String>>
         get() {
             val buildDirFile = projectLayout.buildDirectory.asFile.get()
-            return friendPaths
-                .filter { it.exists() }
-                .map { it.normalize().relativeTo(buildDirFile).invariantSeparatorsPath }.toSet()
+            return friendPaths.elements.map { providedSet ->
+                providedSet
+                    .map { it.asFile }
+                    .filter { it.exists() }
+                    .map { it.normalize().relativeTo(buildDirFile).invariantSeparatorsPath }
+                    .toSet()
+            }
         }
 
     @get:Internal