fix missing(), update docs. That said: not useful!

It just echoes back the packagename:

        B:\code\apt>apt missing libtiff-devel
        libtiff-devel

We should either make it do something worthwhile or rip it out. Probably
Jeremy Palmer made this redundant when adding dependency tracking to
install().
cleanup remove(), minor changes only

pep8 whitespace; use "p" instead of overly verbose and overloaded "packagename"
*** install() is broken ***

        B:\code\apt>apt install iconv
        iconv
        error: i not in [curr]
        error: i not installed

Looks like the so called "not useful" missing() I messed with a bit ago is
important after all, and leads to the first real down-the-rabbit-hole mess
encountered in this round of refactoring. Reminding me that install() REALLY
needs some careful attention and documenting so it's easier to understand
what the heck it's doing.