right trimming for dzil autoprereq plugin

autoprereq plugin for dzil used to trim all modules under the dist namespace from the list of prereqs. eg, for dist-zilla dist, it would trim all dist::zilla::* modules.

although good for a first try, it had two problems:

• modules outside of your dist can be trimmed. eg, audio::mpd is requiring some audio::mpd::common modules which are outside of audio-mpd dist.
• if your dist is shipping 2 namespaces, only the modules inside the first namespace are trimmed, and you can end up with a dist with circular deps.
  

the first one is easy to solve with a fixed [Prereq] section in your dist.ini (this is what i've done for audio-mpd). and the second is not really common, so i thought that i could leave dzpa as is...

but dams now uses dzil for the excellent curses-toolkit - which ships curses::toolkit::* and poe::component::curses::* modules. so he complained (rightly) and i had to fix autoprereq plugin.

so, dzpa will only trim modules that are part of the dist, instead of blindly removing the modules under the dist namespace. and this only change fixes at once the 2 problems outlined above. where dzpa used to trim too much and not enough, it now trims just what it has to...