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/testdata/pointer/fail_deref_not_pointer.carbon

@@ -27,6 +27,7 @@ fn Deref(n: i32) {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace {.Deref = %Deref}
 // CHECK:STDOUT:   %Deref: <function> = fn_decl @Deref
 // CHECK:STDOUT: }
 // CHECK:STDOUT: