cmd/go: enable lazy loading

This change activates the dormant “lazy loading” codepaths added in CL
265777 and its predecessors. Dependencies of modules that declare 'go
1.17' or higher are loaded lazily, and the dependencies in the go.mod
file maintain additional invariants to support more efficient lazy
loading for downstream dependent modules.

See https://golang.org/design/36460-lazy-module-loading for the
detailed design.

For #36460

Change-Id: Ic12ee7842aef9580357fcf8909d87654fcb2ad12
Reviewed-on: https://go-review.googlesource.com/c/go/+/314634


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>