Build: introduce javaPluginConvention extension on project

`ExtensionAware.the<T>()` introduced in gradle 4.7 made existing calls `the<JavaPluginConvention>()` invalid (on wrong receiver)

libraries/kotlinx-metadata/jvm/build.gradle.kts

@@ -49,7 +49,7 @@ val shadowJar = task<ShadowJar>("shadowJar") {
     callGroovy("manifestAttributes", manifest, project)
     manifest.attributes["Implementation-Version"] = version
 
-    from(the<JavaPluginConvention>().sourceSets["main"].output)
+    from(javaPluginConvention().sourceSets["main"].output)
     exclude("**/*.proto")
     configurations = listOf(shadows)