cmd/pack: fix c command for existing file
There were at least two bugs:
1) It would overwrite a non-archive.
2) It would truncate a non-archive and then fail.
In general the file handling was too clever to be correct.
Make it more straightforward, doing the creation
separately from archive management.

Fixes issue 8369.

LGTM=adg, iant
R=golang-codereviews, adg, iant
CC=golang-codereviews
https://codereview.appspot.com/147010046