MavenNebulaPublishPlugin: support java platform plugin publications

[rpalcolea]

Today, folks using our plugin with java-platform need to do the following to publish a BOM:

class PublishMavenPlatformPlugin implements Plugin<Project> {

    @Override
    void apply(Project project) {
        project.plugins.apply(MavenPublishPlugin)
        project.afterEvaluate {
            project.extensions.findByType(PublishingExtension).with {
                publications { PublicationContainer pc ->
                    pc.removeAll()
                    pc.getByName('nebula') { MavenPublication p ->
                        p.from(project.components.getByName('javaPlatform'))
                        p.versionMapping { VersionMappingStrategy v ->
                            v.allVariants { VariantVersionMappingStrategy vv ->
                                vv.fromResolutionResult()
                            }
                        }
                    }
                }
            }
        }
    }
}

This should take care of this boilerplate