cmd/go: in 'go list -m', print effective go.mod file
When the -modfile flag is in use (either explicitly or from GOFLAGS),
'go list -m' will now print the effective go.mod file for the main
module in the GoMod field in -f or -json output.
Fixes #36220
Updates #34506
Change-Id: I89c2ee40f20e07854bb37c6e4e13eeea0cce7b0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/212100
Run-TryBot: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>