internal/txtar: replace with golang.org/x/tools/txtar

Change-Id: I31580c355feee65b3c63b3fe977f216070ce8a21
Reviewed-on: https://go-review.googlesource.com/c/mod/+/207907
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
5 files changed