cmd/go/internal/modfetch: inject Fetcher_ into TidyGoSum

cmd/go/internal/modfetch: inject Fetcher_ into TidyGoSum

This commit begins the injection of the global Fetcher_ variable into
the various function calls that make use of it.  The purpose is to
prepare for the eventual removal of the global Fetcher_ variable and
eliminate global state.

[git-generate]
cd src/cmd/go/internal/modfetch
rf '
  inject Fetcher_ TidyGoSum
  mv haveModSumLocked.fetcher_ haveModSumLocked.f
  mv addModSumLocked.fetcher_ addModSumLocked.f
  mv tidyGoSum.fetcher_ tidyGoSum.f
  mv sumInWorkspaceModulesLocked.fetcher_ sumInWorkspaceModulesLocked.f
'

Change-Id: Iecf736f17d6e63c856355284d09b7982dc9e16b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/724240
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>