Harmonize covers

[rbeezer]

LaTeX/PDF covers are in docinfo, EPUB covers are in the publisher file. LaTeX specification should move to the publisher file, and maybe then consolidate them both. And assume the managed directory scheme (i.e. do not support not having this specified).

<covers>
    <latex front="..." back="..."/>
    <epub  front="..."/>
</covers>

A back cover for EPUB is not much of a thing, but we'll have that option down the road.

New publisher variables, deprecations, etc. necessary.