build(app): pass Kotlin classes compiled by AGP to the metadata generator

With built-in Kotlin, AGP writes the Kotlin classes to
intermediates/built_in_kotlinc/<buildType>/<compile task>/classes rather
than KGP's tmp/kotlin-classes/<buildType>. buildMetadata only scanned the
latter, so app and plugin Kotlin classes were missing from the metadata
and 31 Kotlin specs in the test app failed. Both locations are scanned,
so a project module still built by KGP keeps working.

Author: edusperoni

test-app/app/build.gradle

@@ -833,6 +833,11 @@ task buildMetadata(type: BuildToolTask) {
         inputs.dir(kotlinClassesDir)
     }
 
+    def builtInKotlinClassesDir = layout.buildDirectory.dir("intermediates/built_in_kotlinc").get().asFile
+    if (builtInKotlinClassesDir.exists()) {
+        inputs.dir(builtInKotlinClassesDir)
+    }
+
     outputs.files("$METADATA_OUT_PATH/treeNodeStream.dat", "$METADATA_OUT_PATH/treeStringsStream.dat", "$METADATA_OUT_PATH/treeValueStream.dat")
 
     workingDir "$BUILD_TOOLS_PATH"
@@ -848,6 +853,7 @@ task buildMetadata(type: BuildToolTask) {
 
         def classesSubDirs = []
         def kotlinClassesSubDirs = []
+        def builtInKotlinClassesDirs = []
 
         subprojectBuildDirectories.each { buildDirectory ->
 
@@ -871,6 +877,15 @@ task buildMetadata(type: BuildToolTask) {
                     }
                 }
             }
+
+            // Kotlin compiled by AGP itself lands in <buildType>/<compile task>/classes.
+            def builtInKotlinBuildTypeDir = buildDirectory.dir("intermediates/built_in_kotlinc/$selectedBuildType").get().asFile
+            builtInKotlinBuildTypeDir.listFiles()?.each { compileTaskDir ->
+                def compiledClassesDir = new File(compileTaskDir, "classes")
+                if (compiledClassesDir.isDirectory()) {
+                    builtInKotlinClassesDirs.add(compiledClassesDir)
+                }
+            }
         }
 
         def generatedClasses = new LinkedList<String>()
@@ -886,6 +901,10 @@ task buildMetadata(type: BuildToolTask) {
             }
         }
 
+        for (File compiledKotlinDir : builtInKotlinClassesDirs) {
+            generatedClasses.add(compiledKotlinDir.getAbsolutePath())
+        }
+
         def store = new ArrayList<File>()
         for (String dir : generatedClasses) {
             listf(dir, store)