Refactor load-asd to perform a define-op
Split asdf/system-registry away from asdf/find-system: the former defines the internal *defined-systems* table and the search mechanism, whereas the latter keeps the find-system methods, but also defines a new define-op operation, on load-asd will call operate. Reorder dependencies accordingly. Have forcing come from the plan, so as to not force again actions from dependencies that were already performed in a subcall to operate. Accept asdf.asd from the same version, so asdf.asd doesn't get removed at the very next call to operate (which may now come implicitly at the least provocation).
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