dzil authordeps - There WAS a solution already!
If you've read my last post, there's a good chance you were relieved that there was finally a command to install all the missing plugins.
However, about 5 minutes (if not less) after posting my new thingamajig, Florian Ragwitz posted a reply saying "uh.. it's already been done, and a new Dist::Zilla is released with it today".
So, take even a more relieved breath because Dist::Zilla now already has it. Download it and use `dzil authordeps`!
On a technical note, it was interesting to see a different implementation of the same idea. What Florian did was to simply read the file, expand package names using Dist::Zilla::Util and print them all out. What I tried to do was perhaps more ambitious (but probably an overkill). I ran a dist building process and hooked up to the sections reading method and if the section was a plugin that didn't exist, I didn't run the section reading and marked it down. At the end I had all the sections that were plugins that didn't exist. I like Florian's method better, though. :)