add hidden config to set packages as Essential/Important

add hidden config to set packages as Essential/Important

You can pretty much achieve the same with a local dummy package if you
want to, but libapt has an inbuilt setting for essential: "apt" which
can be overridden with this option as well – it could be helpful in
quick tests and what not so adding this alternative shouldn't really
hurt much.

We aren't going to document them much through as care must be taken in
regards to the binary caches as they aren't invalidated by config
options alone, so the effects of old settings could still be in them,
similar to the other already existing pkgCacheGen option(s).

Closes: 767891
Thanks: Anthony Towns for initial patch