This fixes 2 flaws in the asdf-build-system:

1. Use asdf:load-system instead of asdf:compile-system.  The latter is not
recommended by the manual and fails with at least 2 systems.

2. Add the build directory to the ASDF registry so that .asd files are
automatically found.  This has a double benefit:
  - It dramatically simplifies package definition writing.
  - It fixes a bug which used to cause the check phase to fail.

All commits after the first two adapt the Common Lisp package definitions to
the new build system and in particular fix many check phases.