Reorder MPP plugin logic to fix eager publishing configuration

In Gradle 5.0, the `publishing` extension is configured eagerly, so
we need to make sure everything we use in publishing setup is already
available at the point when it's triggered:

* the `metadata` target
* the source JARs

Issue #KT-28520 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/NewMultiplatformIT.kt

@@ -188,6 +188,18 @@ class NewMultiplatformIT : BaseGradleIT() {
         }
     }
 
+    @Test
+    fun testMavenPublishAppliedBeforeMultiplatformPlugin() =
+        with(Project("sample-lib", GradleVersionRequired.AtLeast("5.0"), "new-mpp-lib-and-app")) {
+            setupWorkingDir()
+
+            gradleBuildScript().modify { "apply plugin: 'maven-publish'\n$it" }
+
+            build {
+                assertSuccessful()
+            }
+        }
+
     @Test
     fun testResourceProcessing() = with(Project("sample-lib", gradleVersion, "new-mpp-lib-and-app")) {
         val targetsWithResources = listOf("jvm6", "nodeJs", "wasm32", nativeHostTargetName)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/KotlinMultiplatformPlugin.kt

@@ -80,14 +80,15 @@ class KotlinMultiplatformPlugin(
         configureSourceSets(project)
 
         setUpConfigurationAttributes(project)
-        configurePublishingWithMavenPublish(project)
-        configureSourceJars(project)
 
         // set up metadata publishing
         targetsFromPreset.fromPreset(
             KotlinMetadataTargetPreset(project, instantiator, fileResolver, kotlinPluginVersion),
             METADATA_TARGET_NAME
         )
+        configureSourceJars(project)
+
+        configurePublishingWithMavenPublish(project)
 
         // propagate compiler plugin options to the source set language settings
         setupCompilerPluginOptions(project)