Our goal is to have simple, minimal jlink API for plugins and hide everything else as internal implementation detail. This issue is a first step towards the goal.

* Only exported package from jdk.jlink module should be jdk.tools.jlink.plugin -- all other packages should be internal.

* All plugin API should be interfaces except for PluginException class

* The following class name renamings are proposed:

   Pool -> ModulePool
   Pool.Module -> LinkModule
   Pool.ModuleData -> ModuleEntry
   Pool.ModuleDataType -> ModuleEntry.Type
   Plugin.CATEGORY -> Plugin.Category
   Plugin.STATE -> Plugin.State

* remove PluginContext and PluginContextImpl

* avoid read-only Collections and null return value from plugin API interfaces. Use Streams and Optional (eg. modules, entries, findModule, findEntry) uniformly.

* Change ReleaseInfoPlugin is a TransformerPlugin rather than a post-processing plugin