void hasPlugin(String alias, @DelegatesTo(strategy = Closure.DELEGATE_FIRST, value = PluginSpec) Closure<Void> spec) {
        assert model.hasPlugin(alias)
        def plugin = model.getPlugin(alias)
        assert plugin != null : "Expected a plugin with alias '$alias' but it wasn't found"
        def pluginSpec = new PluginSpec(plugin)
        spec.delegate = pluginSpec
        spec.resolveStrategy = Closure.DELEGATE_FIRST
        spec()
    }

private
fun pluginSpecsFrom(pluginDescriptorsClassPath: ClassPath): Sequence<PluginTree.PluginSpec> =
    pluginDescriptorsClassPath
        .asFiles
        .asSequence()
        .filter { it.isFile && it.extension.equals("jar", true) }
        .flatMap { pluginEntriesFrom(it).asSequence() }
        .map { PluginTree.PluginSpec(it.pluginId, it.implementationClass) }


private