Accept strings as OPERATION class designators, read in package ASDF.

Refactor COMPONENT class designators to use the same general method.
As a substrate, goodbye uiop/utility:find-class*, hello uiop/utility:coerce-class.

In the operate upgrade handler, assume there is
no uninterning and renaming packages away anymore going forward.

Add tests for strings as class designators using defsystem-depends-on.