cmd/go/internal/modload: implement lazy loading

For #36460
Updates #41297

Change-Id: I1b82176a45df499e52f1a3a0ffe23eab2a1ca86e
Reviewed-on: https://go-review.googlesource.com/c/go/+/265777
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Bryan C. Mills