cmd/go: use local state object in pkg `modcmd`

cmd/go: use local state object in pkg `modcmd`

This commit modifies `modcmd.runDownload` to construct a new
modload.State object using the new constructor instead of the current
global `modload.LoaderState` variable.

This commit is part of the overall effort to eliminate global
modloader state.

[git-generate]
cd src/cmd/go/internal/modcmd
rf '
  add download.go:/func runDownload\(/-0 var moduleLoaderState *modload.State
  ex {
    import "cmd/go/internal/modload";
    modload.LoaderState -> moduleLoaderState
  }
  add runDownload://+0 moduleLoaderState := modload.NewState()
  add runEdit://+0 moduleLoaderState := modload.NewState()
  add runGraph://+0 moduleLoaderState := modload.NewState()
  add runInit://+0 moduleLoaderState := modload.NewState()
  add runTidy://+0 moduleLoaderState := modload.NewState()
  add runVendor://+0 moduleLoaderState := modload.NewState()
  add runVerify://+0 moduleLoaderState := modload.NewState()
  add runWhy://+0 moduleLoaderState := modload.NewState()
  rm download.go:/var moduleLoaderState \*modload.State/
'

Change-Id: Id69deba173032a4d6da228eae28e38bd87176db5
Reviewed-on: https://go-review.googlesource.com/c/go/+/711125
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>