allow for undefined component-class when disabled by :if-feature

[Faré]

I tried to use the following components:
  (sb-grovel:grovel-constants-file "sbcl/constants"
      :package :com.dvlsoft.clon :if-feature :sbcl)
  (:grovel-file "cffi/constants" :if-feature (:or :allegro :clisp :lispworks))
  (:file "termio")

But
(1) the first one doesn't work without a #+sbcl, because the package and class aren't present outside of asdf. The result would be the same putting the class name in a string, except that this doesn't look like it's even working at all.

(2) the second one doesn't work if I conditionally system-depends-on (or allegro clisp lispworks), for then asdf still tries to instantiate a class, and fails.

(3) if you try to use (:feature :sbcl (:require "sb-grovel")) and/or (:feature (:or :allegro :clisp :lispworks) "cffi-grovel")) in your :system-depends-on, load-systems* will bork when trying to load them and they are expanded to NIL instead of a valid system name, unless once again you use #+ to protect the form, which defeats the goal of avoiding #+.

FEATURE REQUEST:
* have some kind of "invalid-component" class that ASDF uses in such cases, whereby no error happens if :if-feature causes asdf to avoid the component, but error happens if you actually try to depend on the component without it being nulled out by :if-feature.
* suitably expand missing classes into said invalid-component.
* an invalid component has a message slot to explain where the missing component comes from.
* remove nil's before to call load-systems* and/or inside load-systems*

(Issues found while trying to cleanup the build of Didier Verna's clon.)

[Faré (fahree)]

Point 3 and its solution are now https://bugs.launchpad.net/asdf/+bug/1445638