Incremental defsystem dependencies

[Faré]

Dear Stelian, dear Lispers,

> in Gitlab MR !37 I try to address some fundamental bugs regarding the
> proper treatment of incrementality for defsystem dependencies. See for
> instance the test I just added, that still fails at this time,
> test-defsystem-depends-on-change.script.
> https://gitlab.common-lisp.net/asdf/asdf/merge_requests/37
>
The test now passes, and !37 is working, though I'll probably cleanup
the code some before I mark it WIP no more.

> The bad news is that it becomes even harder than before to support a
> system that has defsystem-depends-on on another system defined in the
> very same file, a feature currently relied upon by iolib.
>
And indeed !37 fails to compile iolib, for this reason. Glancing at
Quicklisp, iolib might be the only system affected (but I'd need a
better test, and many systems fail to load on my machine due to CFFI
being unable to find some .so, which is about any system depending on
any .so, since I'm using NixOS).

> Stelian, how sad would you be if you have to segregate
> defsystem-depends-on to refer to systems in separate files?
>
Stelian, what do you think?

I could try to *also* implement deferred evaluation of defsystem
forms, but that would be a pain: now ASDF would need an additional
registry, for unevaluated defsystem forms.

—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org
There's no worse blasphemy than to claim authority on telling what pleases or
displeases God. Hence prophets and priests are the most impious men on earth.
               — Faré