repoman: add a mini framework for checking eclasses, and fill it out

repoman: add a mini framework for checking eclasses, and fill it out

Rather than copying & pasting the same behavior for the different eclass
checks, add a common class for them to extend.  This makes adding more
eclass checks trivial, and keeps down bitrot.

This does abuse the checking interface slightly -- the eclass will change
its category between unused and missing based on the checks.

URL: https://bugs.gentoo.org/417159
URL: https://bugs.gentoo.org/417231
Signed-off-by: Mike Frysinger <vapier@gentoo.org>