internal/pkgbits: add version number

Especially once this code gets copied into x/tools, we need a way to
evolve the file format, so add an explicit version number.

Change-Id: I9cc2e357c3ca3f07fd8d0c0ba4e4a95f89edeac6
Reviewed-on: https://go-review.googlesource.com/c/go/+/388914


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>