MixinGradle maven repository doesn't have Plugin Marker Artifacts

[Niko-sk2x]

Same issue as MinecraftForge/ForgeGradle#508

[NikkyAI]

this can be easily fixed by using the java-gradle-plugin and maven-publish gradle plugins, they register publish tasks that will also publish the marker

https://docs.gradle.org/4.10-rc-2/userguide/java_gradle_plugin.html#example_using_the_gradleplugin_block