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.

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.

On Allegro, fix the way we avoid autoloading ASDF.
But more verbose when versioning goes amiss.
Mark source-file-type as redefined to avoid issues with type override when upgrading.

Ernst's test case and ours were not the same.

Ernst van Waning reports that the return value of ASDF:COMPONENT-PATHNAME
does not honor an override of SOURCE-FILE-TYPE.  This test verifies the
claim.