More refactoring of plan
Move action-status wholly to plan, merge it with planned-action-status. Add a performable-p flag to plans. Simplify sequential-plan and required-components; have them use a separate function collect-action-dependencies for traversing the graph. Add a function action-up-to-date-p to check whether an action is up-to-date, without adding it to the plan if it wasn't.
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