Put \ifx guards on most other package includes, too.

We shouldn't be trying to load "amsopn" ten times, for example. But
testing whether or not a package is loaded is tricky. First, I tried
the \@ifpackageloaded macro: that causes problems with some of my
existing documents, and the "spacefactor" error that results is
apparently a FAQ. But I don't know what to do about it.

The other way to test if a package is loaded is by checking for some
(unique?) macro that it defines. That's what I've guessed at here.  It
works in most cases, like when we want \operatorname from "amsopn."
But, some packages also didn't have something obvious to check for, so
coverage isn't 100%.