cmd/go: with -mod=vendor, don't panic if there are duplicate requirements

In loadModFile with -mod=vendor, load the vendor list and use it to
initialize the module graph before calling updateRoots.

In updateLazyRoots with any mode other than "mod", return the original
*Requirements if no roots needed to be upgraded, even if there are
inconsistencies. This means 'go list -m -mod=readonly' and -mod=vendor
may succeed if there are duplicate requirements or requirements on
versions of the main module.

Fixes #47565

Change-Id: I4640dffc4a7359367cc910da0d29e3538bfd1ca4
Reviewed-on: https://go-review.googlesource.com/c/go/+/340252


Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>