DefaultPluginDescriptorCache does not retain pluginDescriptor dependencies

PluginDescriptor dependencies is defined in META-INF/maven/plugin.xml, so there is no reason not to retain this field when storing and retrieving PluginDescriptor instances to and from descriptor cache. The attribute can be used in embedding scenarios and for command line builds to quickly determine if plugin has certain dependencies or not, without having to fully resolve plugin dependencies. I'll commit a fix with corresponding unit test shortly.