- `parse_anonymous_spec()` is a vestige of the days when Spack didn't
support nameless specs. We don't need it anymore because now we can
write Spec() for a spec that will match anything, and satisfies()
semantics work properly for anonymous specs.
- Delete `parse_anonymous_spec()` and replace its uses with simple calls
to the Spec() constructor.
- make then handling of when='...' specs in directives more consistent.
- clean up Spec.__contains__()
- refactor directives and tests slightly to accommodate the change.
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