In particular, the Property and DeadPropsHolder types need to refer to
the standard xml package, not the internal fork, to be usable by other
packages.

Inside the package, the XML marshaling and unmarshaling is still done by
the etc/internal/xml package, and will remain that way until
https://github.com/golang/go/issues/13400 is resolved.

Fixes golang/go#15128.

Change-Id: Ie7e7927d8b30d97d10b1a4a654d774fdf3e7a1e3
Reviewed-on: https://go-review.googlesource.com/21635Reviewed-by: Andrew Gerrand <adg@golang.org>