In switching to hash facts for concrete specs, we lost the transitive facts
from dependencies. This was fine for solves, because they were implied by
the imposed constraints from every hash. However, for `spack diff`, we want
to see what the hashes mean, so we need another mode for `spec_clauses()` to
show that.
This adds a `expand_hashes` argument to `spec_clauses()` that allows us to
output *both* the hashes and their implications on dependencies. We use
this mode in `spack diff`.
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