* Simplified Spec.__init__ signature by removing the *dep_like argument.
The `*dep_like` argument of `Spec.__init__` is used only for tests. This
PR removes it from the call signature and introduces an equivalent
fixture to be used in tests.
* Refactored ``spec_from_dict`` to be a static method of ``Spec``
The fixture ``spec_from_dict`` has been refactored to be a static method
of ``Spec``. Test code has been updated accordingly. Added tests for
exceptional paths.
* Renamed argument `unique` to `normal` + added LazySpecCache class
As requested in the review the argument `unique` of `Spec.from_literal`
has been renamed to `normal`. To avoid eager evaluations of
`Spec(spec_like)` expressions a subclass of `collections.defaultdict`
has been introduced.
* Spec object can be keys of the dictionary for a spec literal.
Added back the possibility use a spec directly as a key. This permits
to build DAGs that are partially normalized.
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