cmd/go: move 'go install cmd@version' code into internal/load

'go run cmd@version' will use the same code.

This changes error messages a bit.

For #42088

Change-Id: Iaed3997a3d27f9fc0e868013ab765f1fb638a0b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/310410


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>