cmd/go: don't save sums for modules loaded for import resolution

modfetch.WriteGoSum now accepts a map[module.Version]bool parameter.
This is used to prevent some new sums from being saved to go.sum when
they would be removed by the next 'go mod tidy'. Previusly, sums were
saved for modules looked up during import resolution.

A new function, modload.TrimGoSum, is also introduced, which marks
sums for deletion. 'go mod tidy' now uses this. The new logic
distinguishes between go.mod sums and content sums, which lets 'go mod
tidy' delete sums for modules in the build graph but not the build
list.

Fixes #31580
Fixes #36260
Fixes #33008

Change-Id: I06c4125704a8bbc9969de05265967ec1d2e6d3e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/237017


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>