cmd/go: `go mod download -json` is a lot slower with go1.13 [1.13 backport]

[gopherbot]

@bcmills requested issue #34533 to be considered for backport to the next 1.13 minor release.

@gopherbot, please backport to Go 1.13.2: this feature caused a significant performance regression for large repositories, and was newly introduced and fairly obscure (so is unlikely to have broad usage at this point).

The only likely users are other module proxy implementations; we're asking them explicitly if they care about this field in a golang-module-proxies thread.

[gopherbot]

Change https://golang.org/cl/199079 mentions this issue: [release-branch.go1.13] Revert "cmd/go: add a Latest field to the output of 'go mod download -json'"

[gopherbot]

Closed by merging cf54026 to release-branch.go1.13.