text: copy exported packages to destination repo
text: copy exported packages to destination repo

Also rewrote vendoring update code to use the same code.
This way vendored packages don’t have to be manually
updated if there are new files to be added.

Change-Id: Ifcb48148cbe7badbc1a37eadf96ec4b975028f83
Reviewed-on: https://go-review.googlesource.com/34649
Run-TryBot: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/text