cmd/go: modules discarded during resolution should not be added to go.sum

[rsc]

If I put a bad import into a program and run "go build",
then it looks in various modules for the import,
none of which work, but all of which end up in go.sum.
They should not.

[julieqiu]

/cc @bcmills @jayconrod

[gopherbot]

Change https://golang.org/cl/237017 mentions this issue: cmd/go: don't save sums for modules loaded for import resolution