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/nested_const.carbon

@@ -12,6 +12,7 @@ fn F(p: const (const (const i32*)*)) -> const i32 {
 // CHECK:STDOUT: --- nested_const.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace {.F = %F}
 // CHECK:STDOUT:   %F: <function> = fn_decl @F
 // CHECK:STDOUT: }
 // CHECK:STDOUT: