Add support for 'package' in expressions. (#3445)

This creates a namespace for `package` scope.

It looks like names of class_decls in namespaces lead to an unexpected
instref. This is already true, as best as I can tell. I'm not sure if
there's a preferred approach to address that, so I've left a TODO for
now.

toolchain/check/node_stack.h

@@ -303,6 +303,7 @@ class NodeStack {
       case Parse::NodeKind::InfixOperator:
       case Parse::NodeKind::MemberAccessExpr:
       case Parse::NodeKind::NameExpr:
+      case Parse::NodeKind::PackageExpr:
       case Parse::NodeKind::ParenExpr:
       case Parse::NodeKind::PostfixOperator:
       case Parse::NodeKind::PrefixOperator: