Tests: SBCL passes test-lisp and test-upgrade
Also:
* big refactoring of test infrastructure
* contrib/debug.lisp and (asdf/driver:asdf-debug) for debugging.
* remove aif and it, use the cleaner if-bind.
* backtrace support, with improvements from trivial-backtrace.
* integration of asdf-condition-control (originally from xcvb-driver)
* simplify the upgrade heuristic: always tries to upgrade once.
  The previous heuristic was too complex with too many failure cases.
  You are thus assumed to want a given version asdf
  if and only if it is in your source-registry.

This restores the feature of do-first that was lost while refactoring ASDF
to fix the timestamp propagation issue.

The old asdf-bundle worked implicitly because of do-first:
fasl-op DEPENDS-ON compile-op, and compile-op has a DO-FIRST on the
load-op of dependencies, NOT a DEPENDS-ON, so if the compile-op is
"done" (which did NOT check the timestamps of dependencies),
then the dofirst is never consulted and load-op is never traversed.
In our new ASDF, we want to always traverse the load-op of dependencies,
so we can propagate their timestamps; however, we still want to only
actually load them if they are needed. Therefore, visiting actions
takes an additional flag "needed in this image", and each visited action
can be planned or not planned yet, and a not planned yet action can
be visited a second time to plan it after recursively visiting its
dependencies with the "needed" flag set.

Tests were fixed now to work now that this distinction exists.

where it ought to always have been, based on a properly cleaned up prepare-op
(previously introduced in 2.26.14 as parent-load-op).

This is a checkpoint of a mostly working system after major changes;
it requires further cleanup and fixes:
strangely, fails some tests on CCL but not SBCL.

Introduce find-operation as a companion to find-component,
subsuming make-sub-operation.
The results of component-depends-on are processed with find-operation and resolve-component-spec.
component-self-dependencies is fixed to play well with prepare-op.

Also, some internal renamings and signature changes:
parent-operation becomes upward-operation
parent-load-op becomes prepare-op
parent-source-load op becomes prepare-source-op
parent-component is distinguished from module
module-components becomes component-children
module-components-by-name becomes component-children-by-name
compute-module-components-by-name becomes compute-children-by-name
circular-dependency-components becomes circular-dependency-actions
component-load-dependencies becomes component-sibling-dependencies (with a stub)
introducing child-component, downward-operation.
refactoring visit-dependencies to include the loop in which it's always used.
refactoring force and force-not processing to use a function action-override-p;
force is achieved with a trivial method on operation-done-p for prepare-op,
and force-not as a trivial check in visit-action.

Tweak tests along the way. Tweak the documentation.

Failed to either rename operation-error or introduce action-error in its superclasses:
that breaks sb-grovel.

Use probe-file for probe-file* in LispWorks.

Make ABCL's jar translation happier with
the recent compile-file-pathname* change for Stelian by redoing cfp myself.

Make truenamize work when the merged pathname is relative.
Makes it happier when e.g. the *default-pathname-defaults* is #p"",
which is the case notably on CCL.

Try and fail to support GCL 2.6 and have 2.7 fail less;
too many bugs, in CONDITIONs, PATHNAMEs, and more,
plus compiler bug that breaks coerce-pathname.

Also, use coerce-pathname in asdf-ecl and deprecate merge-component-name-type.

Simplify test scripts.

perl -pi.bak -e 's|"../asdf"|\(compile-file-pathname "../asdf"\)|;' *.script

* made it easier to debug tests. export DEBUG_ASDF_TEST=t
* add ECL support to tests: passes all 18 scripts, but have to ignore compile warnings.
* lots of whitespace issues, including ones that made .gitignore not work!

Found that the current load recipe (load "../asdf") in the test
scripts was brittle, because the presence of asdf.fasl was obscured
if there was an "asdf/" directory in ..  Unfortunately, the result
of makeinfo --html asdf.texinfo created just such a directory!
fe[nl]ix kindly provided a pointer to compile-file-pathname that
solved the problem.