Don’t call spec.format in Database._get_matching_spec_key (#38792)

`"%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__`

lib/spack/spack/database.py

@@ -1216,7 +1216,7 @@ def _get_matching_spec_key(self, spec, **kwargs):
             match = self.query_one(spec, **kwargs)
             if match:
                 return match.dag_hash()
-            raise KeyError("No such spec in database! %s" % spec)
+            raise NoSuchSpecError(spec)
         return key
 
     @_autospec
@@ -1672,3 +1672,17 @@ def __init__(self, database, expected, found):
     @property
     def database_version_message(self):
         return f"The expected DB version is '{self.expected}', but '{self.found}' was found."
+
+
+class NoSuchSpecError(KeyError):
+    """Raised when a spec is not found in the database."""
+
+    def __init__(self, spec):
+        self.spec = spec
+        super().__init__(spec)
+
+    def __str__(self):
+        # This exception is raised frequently, and almost always
+        # caught, so ensure we don't pay the cost of Spec.__str__
+        # unless the exception is actually printed.
+        return f"No such spec in database: {self.spec}"