Revert "cmd/go/internal/modconv: use modules to examine instead of using only direct source control entries"
This reverts CL 191218.
Reason for revert: broke cmd/go/internal/modconv in the 'linux-amd64-longtest' builder.
(https://build.golang.org/log/
e28011d0f918d4b4c503ab47e479d9e76c769abd)
Change-Id: I0d260b0a5ad510d3d304c8aac8286fcab921d2fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/194797
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>