`"%s" % spec` formats the spec with deps included, which produces sometimes KBs
of data and is slow to run in pure Python. It can delay otherwise very short-lived
read/write locks on the database.
Discovered in #38762 where profile output showed about 2 seconds is spent in
`spec.format`, which is significant overhead when using multiprocessing to install
from binary cache in parallel (installation often takes <5s for small packages). With
this change, `spec.format` no longer shows up in profile output.
(This line hasn't changed since Spack v0.9 ;p)
* move format() call to custom NoSuchSpecError exception
* add a comment saying why, so we can eventually change `Spec.__str__`
374fda1063