repo.py: drop deleted packages from provider cache (#43779)

The reverse provider lookup may have stale entries for deleted packages, which used to cause errors. It's hard to invalidate those cache entries, so this commit simply drops entries w/o invalidating the cache.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

lib/spack/spack/repo.py

@@ -726,14 +726,14 @@ def first_repo(self):
         """Get the first repo in precedence order."""
         return self.repos[0] if self.repos else None
 
+    @llnl.util.lang.memoized
+    def _all_package_names_set(self, include_virtuals):
+        return {name for repo in self.repos for name in repo.all_package_names(include_virtuals)}
+
     @llnl.util.lang.memoized
     def _all_package_names(self, include_virtuals):
         """Return all unique package names in all repositories."""
-        all_pkgs = set()
+        return sorted(self._all_package_names_set(include_virtuals), key=lambda n: n.lower())
-        for repo in self.repos:
-            for name in repo.all_package_names(include_virtuals):
-                all_pkgs.add(name)
-        return sorted(all_pkgs, key=lambda n: n.lower())
 
     def all_package_names(self, include_virtuals=False):
         return self._all_package_names(include_virtuals)
@@ -794,7 +794,11 @@ def patch_index(self):
 
     @autospec
     def providers_for(self, vpkg_spec):
-        providers = self.provider_index.providers_for(vpkg_spec)
+        providers = [
+            spec
+            for spec in self.provider_index.providers_for(vpkg_spec)
+            if spec.name in self._all_package_names_set(include_virtuals=False)
+        ]
         if not providers:
             raise UnknownPackageError(vpkg_spec.fullname)
         return providers