cmd/go/internal/modload: only check root-promotion during tidy for lazy modules

In a lazy module, it is important that tidyRoots does not add any new
roots because the dependencies of non-roots are pruned out.

In an eager module, that property is not important (and does not hold
in general) because no dependencies are ever pruned out.

Fixes #45952

Change-Id: I5c95b5696b7112b9219e38af04e0dece7fb6e202
Reviewed-on: https://go-review.googlesource.com/c/go/+/316754


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>