ninjabackend: track all outputs using a set

We need to verify that we don't produce multiple rules for the same
file. This uniqueness check is easy to do with a set, but the original
old implementation used a dict with True as the value. This isn't a
terrible way to implement a set -- we do it for our own internal
OrderedSet implementation, even, and back in prehistory (python 2.3) the
standard library's set type was one. But it was deleted and replaced
with a fast native implementation, and we should too.

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