cmd/go/internal/modload: add a "v" prefix to the indexed go version

This allows semver-based comparisons of the version without additional allocations.

Also comment on the reason for the loops that iterate over modFile instead.

(I was reading the vendor code in order to add the lazy-loading version check,
and this section was a bit unclear to me.)

For #36460

Change-Id: I11559d81ffb4eba0e4e10e6fa3c01990b11f9180
Reviewed-on: https://go-review.googlesource.com/c/go/+/240622


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>