Remove InstId::Builtin members (#4632)

- `InstId::Builtin<Inst>` -> `<Inst>::SingletonInstId`
- `InstId::PackageNamespace` -> `Namespace::PackageInstId`

toolchain/check/scope_stack.cpp

@@ -122,7 +122,7 @@ auto ScopeStack::LookupInLexicalScopes(SemIR::NameId name_id)
 
   // If we have no lexical results, check all non-lexical scopes.
   if (lexical_results.empty()) {
-    return {LexicalLookupHasLoadError() ? SemIR::InstId::BuiltinErrorInst
+    return {LexicalLookupHasLoadError() ? SemIR::ErrorInst::SingletonInstId
                                         : SemIR::InstId::Invalid,
             non_lexical_scope_stack_};
   }