Upgrade failures for asdf-3.1.7 and later

[James M. Lawrence]

Starting with a bare lisp image (e.g --no-userinit --no-sysinit for
SBCL), the following signals a "component not found" error:

(load "asdf-2.26.lisp")
;;; or whatever system you want
(push #p"~/quicklisp/dists/quicklisp/software/alexandria-20170227-git/"
      asdf:*central-registry*)
(load "asdf-3.2.1.lisp")
(asdf:load-system "alexandria")

The first bad commit is https://github.com/fare/asdf/commit/3a9457a

I hope this wasn't intentional? Ideally an old asdf version would
never get loaded, but practically speaking it sometimes does (e.g. on
LispWorks PE and CLISP). And if that happens, thereafter one is unable
to load a system that requires asdf-3. The above case is a
whittled-down version of a quickloading problem.

Best,
lmj