Possible incompleteness in INPUT-FILES for DEFINE-OP

[Robert Goldman]

 I think I may have stumbled on an incompleteness in this method.
Here's the test case:

1.  Define a system. S1, with a version loaded from a version file;
2.  Load S1 into session.
3.  Hack hack hack.
4.  Bump S1's version number in the version.lisp-expr file
5.  Load S2 that depends on new version of S1.

As far as I can tell, ASDF is simply checking the .asd file, not
S1-version.lisp-expr, to see if it's stale.  So my load in step 5 is
failing.

This is, I believe, because S1-version.lisp-expr is not in the set of
INPUT-FILES for S1's DEFINE-OP.

It's a little unsettling to change this on the fly, but I think the
right thing is probably to push the version.lisp-expr file onto a set of
ancillary dependencies when it's read.

If this sounds right, I'll stick this in as a ticket, and try to find
where we could inject a fix.

Best,
r