Implement #4864: Core is a keyword (#4909)

Change representation of package names from `IdentifierId` to
`PackageNameId`, and add a special value `PackageNameId::Core` for the
Core package. Add a `Core` expression to name the Core package, and
support for parsing the `Core` keyword in `package` and `import`
declarations.

For now, I've made no changes to instruction fingerprinting or name
mangling. This means that fingerprints and mangled names will collide
between names in the `Core` package and names in a `r#Core` package. See
#4908.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/sem_ir/name_scope.cpp

@@ -91,9 +91,7 @@ auto NameScopeStore::IsCorePackage(NameScopeId scope_id) const -> bool {
   if (!IsPackage(scope_id)) {
     return false;
   }
-  auto scope_name =
-      file_->names().GetAsStringIfIdentifier(Get(scope_id).name_id());
-  return scope_name == "Core";
+  return Get(scope_id).name_id() == NameId::Core;
 }
 
 }  // namespace Carbon::SemIR