- Since early Spack versions, the SpecParser has (weirdly) been
responsible for initializing Spec fields.
- This refactors initialization to take place in Spec.__init__, as it
probably should have originally.
- This makes the code easier to read, the parser easier to understand,
and removes the use of __new__ in the parser to initialize the Spec.
- This also makes it possible to make a completely empty Spec with
`Spec()` -- this is an abstract Spec that will match anything.
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