cmd/go/internal/modfetch: move global goSum to Fetcher_

cmd/go/internal/modfetch: move global goSum to Fetcher_

[git-generate]
cd src/cmd/go/internal/modfetch
rf '
  add Fetcher.sumState mu sync.Mutex
  ex {
      goSum.mu -> Fetcher_.mu
      goSum.m -> Fetcher_.sumState.m
      goSum.w -> Fetcher_.sumState.w
      goSum.status -> Fetcher_.sumState.status
      goSum.overwrite -> Fetcher_.sumState.overwrite
      goSum.enabled -> Fetcher_.sumState.enabled
      goSum.sumState -> Fetcher_.sumState
    }
    rm goSum
'

Change-Id: I3693905d5bed19f7bae60f5bc1ec5097354c9427
Reviewed-on: https://go-review.googlesource.com/c/go/+/722582
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>