Fix compatibility with Gradle < 7.1 (#957)

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/application/internal/configureApplication.kt

@@ -16,10 +16,7 @@ import org.jetbrains.compose.desktop.application.dsl.Application
 import org.jetbrains.compose.desktop.application.dsl.TargetFormat
 import org.jetbrains.compose.desktop.application.internal.validation.validatePackageVersions
 import org.jetbrains.compose.desktop.application.tasks.*
-import org.jetbrains.compose.internal.KOTLIN_JVM_PLUGIN_ID
-import org.jetbrains.compose.internal.KOTLIN_MPP_PLUGIN_ID
-import org.jetbrains.compose.internal.javaExt
-import org.jetbrains.compose.internal.mppExt
+import org.jetbrains.compose.internal.*
 import org.jetbrains.kotlin.gradle.plugin.KotlinPlatformType
 import java.io.File
 import java.util.*
@@ -34,7 +31,7 @@ fun configureApplicationImpl(project: Project, app: Application) {
         if (project.plugins.hasPlugin(KOTLIN_MPP_PLUGIN_ID)) {
             project.configureFromMppPlugin(app)
         } else if (project.plugins.hasPlugin(KOTLIN_JVM_PLUGIN_ID)) {
-            val mainSourceSet = project.javaExt.sourceSets.getByName("main")
+            val mainSourceSet = project.javaSourceSets.getByName("main")
             app.from(mainSourceSet)
         }
     }

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/preview/internal/configurePreview.kt

@@ -3,10 +3,7 @@ package org.jetbrains.compose.desktop.preview.internal
 import org.gradle.api.Project
 import org.jetbrains.compose.desktop.application.dsl.ConfigurationSource
 import org.jetbrains.compose.desktop.preview.tasks.AbstractConfigureDesktopPreviewTask
-import org.jetbrains.compose.internal.KOTLIN_JVM_PLUGIN_ID
-import org.jetbrains.compose.internal.KOTLIN_MPP_PLUGIN_ID
-import org.jetbrains.compose.internal.javaExt
-import org.jetbrains.compose.internal.mppExt
+import org.jetbrains.compose.internal.*
 import org.jetbrains.kotlin.gradle.plugin.KotlinPlatformType
 import org.jetbrains.kotlin.gradle.targets.jvm.KotlinJvmTarget
 
@@ -20,7 +17,7 @@ fun Project.initializePreview() {
         }
     }
     plugins.withId(KOTLIN_JVM_PLUGIN_ID) {
-        val config = ConfigurationSource.GradleSourceSet(project.javaExt.sourceSets.getByName("main"))
+        val config = ConfigurationSource.GradleSourceSet(project.javaSourceSets.getByName("main"))
         registerConfigurePreviewTask(project, config)
     }
 }

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/internal/projectExtensions.kt

@@ -6,7 +6,10 @@
 package org.jetbrains.compose.internal
 
 import org.gradle.api.Project
+import org.gradle.api.plugins.JavaPluginConvention
 import org.gradle.api.plugins.JavaPluginExtension
+import org.gradle.api.tasks.SourceSetContainer
+import org.gradle.util.GradleVersion
 import org.jetbrains.compose.ComposeExtension
 import org.jetbrains.compose.web.WebExtension
 import org.jetbrains.kotlin.gradle.dsl.KotlinMultiplatformExtension
@@ -23,8 +26,9 @@ internal val Project.mppExt: KotlinMultiplatformExtension
 internal val Project.mppExtOrNull: KotlinMultiplatformExtension?
     get() = extensions.findByType(KotlinMultiplatformExtension::class.java)
 
-internal val Project.javaExt: JavaPluginExtension
-    get() = javaExtOrNull ?: error("Could not find JavaPluginExtension ($project)")
-
-internal val Project.javaExtOrNull: JavaPluginExtension?
-    get() = extensions.findByType(JavaPluginExtension::class.java)
+internal val Project.javaSourceSets: SourceSetContainer
+    get() = if (GradleVersion.current() < GradleVersion.version("7.1")) {
+        convention.getPlugin(JavaPluginConvention::class.java).sourceSets
+    } else {
+        extensions.getByType(JavaPluginExtension::class.java).sourceSets
+    }