Rewrite plugin to _not_ be a FileMunger

This fixes a bug where it expected to see a file named xt/author/pod-spell.t
but recent versions of dzil seem to expose this as t/author-pod-spell.t

I wasn't able to come up with a test for this but I did test this with one of
my distros and it fixed the problem I was seeing.

Honestly, I have no idea what exactly changed. Regardless, this is more correct anyway. It's kind of silly to loop through _all_ the files in the distro just to find the one we want to munge. We might as well munge the content _before_ adding it to the list of files in the distro.