allow uninstalled packages to be put on hold

allow uninstalled packages to be put on hold

dpkg wants to know about a package before it can be put on hold, so we
have to at least hint about its existance in the available file it
"maintaince" to know about such stuff. The simple thing would probably
be to just feed all Packages files into dpkg as well, but what would be
the point really? Exactly, so we take a shortcut here and just create
dummies in the available file if we need to which isn't going to be that
common as usually you are holding packages back and not off.

Who would have thought that a simple feature like setting a package on
hold requires more than 200 lines of code… at least with the testcase it
is now explicitly tested code.