Reorder conditions of `needed?`

[jgomo3]

1. Asking for a setting option is less expensive than tree transversal on dependant tasks.
2. out_of_date? should care about "date" concepts, and not concerning about "build_all" option.

Reordering allows us to remove the "build_all" check in the out_of_date, achieving the quality statement in the second item of the previous list.

Suddenly, there is no need to make a distinction between FileTask or other tasks.