Replace ad-hoc encoding of export version info with a
more systematic approach.

Continue to read (but not write) the Go1.7 format for backward-
compatibility. This will avoid spurious errors with old installed
packages.

Fixes #16244.

Change-Id: I945e79ffd5e22b883250f6f9fac218370c2505a2
Reviewed-on: https://go-review.googlesource.com/27452Reviewed-by: Matthew Dempsky <mdempsky@google.com>