cmd/go/internal/modload: avoid loading the full module graph to determine which checksums to add to go.sum
cmd/go/internal/modload: avoid loading the full module graph to determine which checksums to add to go.sum

For #36460

Change-Id: I606314054bd9064f7c4053f56049fabbaec54143
Reviewed-on: https://go-review.googlesource.com/c/go/+/309189
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go