Fix error when KSP plugin applied before AGP

Previously, when the KSP plugin was applied before AGP and AGP's
built-in Kotlin support for test fixtures or screenshot tests was
enabled, there would be an error when building.

The underlying cause for this error is
gradle/gradle#31092, which caused the KSP
plugin to try to access the android extension before AGP's apply method
had completed.

This change fixes the problem by using the "com.android.base" plugin as
the indicator that a module is an Android module. The "com.android.base"
plugin is applied after the android extension has been added, so it
works around the Gradle bug.

Bug: #2174
Test: GradleCompilationTest

gradle-plugin/src/main/kotlin/com/google/devtools/ksp/gradle/AndroidPluginIntegration.kt

@@ -39,14 +39,10 @@ import java.util.concurrent.Callable
 @Suppress("UnstableApiUsage") // some android APIs are unsable.
 object AndroidPluginIntegration {
 
-    private val agpPluginIds = listOf("com.android.application", "com.android.library", "com.android.dynamic-feature")
-
     fun forEachAndroidSourceSet(project: Project, onSourceSet: (String) -> Unit) {
-        agpPluginIds.forEach { agpPluginId ->
-            project.pluginManager.withPlugin(agpPluginId) {
-                // for android apps, we need a configuration per source set
-                decorateAndroidExtension(project, onSourceSet)
-            }
+        project.pluginManager.withPlugin("com.android.base") {
+            // for android modules, we need a configuration per source set
+            decorateAndroidExtension(project, onSourceSet)
         }
     }
 

gradle-plugin/src/test/kotlin/com/google/devtools/ksp/gradle/GradleCompilationTest.kt

@@ -414,4 +414,16 @@ class GradleCompilationTest {
         )
         testRule.runner().withArguments(":app:assembleDebug").build()
     }
+
+    /**
+     * Regression test for https://github.com/google/ksp/issues/2174
+     */
+    @Test
+    fun androidGradlePluginBuiltInKotlinWithKspAppliedFirst() {
+        testRule.setupAppAsAndroidApp(applyKspPluginFirst = true)
+        // Enable AGP's built-in Kotlin support for test fixtures
+        testRule.runner()
+            .withArguments("tasks", "-Pandroid.experimental.enableTestFixturesKotlinSupport=true")
+            .build()
+    }
 }

gradle-plugin/src/test/kotlin/com/google/devtools/ksp/gradle/testing/KspIntegrationTestRule.kt

@@ -93,14 +93,25 @@ class KspIntegrationTestRule(
     /**
      * Sets up the app module as an android app, adding necessary plugin dependencies, a manifest
      * file and necessary gradle configuration. If [enableAgpBuiltInKotlinSupport] is true, enable AGP's built-in Kotlin
-     * support instead of applying the Kotlin Android Gradle plugin.
+     * support instead of applying the Kotlin Android Gradle plugin. If [applyKspPluginFirst] is true, apply the KSP
+     * plugin first.
      */
-    fun setupAppAsAndroidApp(enableAgpBuiltInKotlinSupport: Boolean = false) {
+    fun setupAppAsAndroidApp(
+        enableAgpBuiltInKotlinSupport: Boolean = false,
+        applyKspPluginFirst: Boolean = false
+    ) {
         testProject.appModule.plugins.addAll(
-            listOf(
-                PluginDeclaration.id("com.android.application", testConfig.androidBaseVersion),
-                PluginDeclaration.id("com.google.devtools.ksp", testConfig.kspVersion)
-            )
+            if (applyKspPluginFirst) {
+                listOf(
+                    PluginDeclaration.id("com.google.devtools.ksp", testConfig.kspVersion),
+                    PluginDeclaration.id("com.android.application", testConfig.androidBaseVersion)
+                )
+            } else {
+                listOf(
+                    PluginDeclaration.id("com.android.application", testConfig.androidBaseVersion),
+                    PluginDeclaration.id("com.google.devtools.ksp", testConfig.kspVersion)
+                )
+            }
         )
         if (enableAgpBuiltInKotlinSupport) {
             testProject.appModule