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

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

This commit modifies package `workcmd` 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/workcmd
rf '
  inject modload.LoaderState runUse
  add sync.go:/func runSync\(/-0 var moduleLoaderState *modload.State
  ex {
    import "cmd/go/internal/modload";
    modload.LoaderState -> moduleLoaderState
  }
  add runSync://+0 moduleLoaderState := modload.NewState()
  add runEditwork://+0 moduleLoaderState := modload.NewState()
  add runInit://+0 moduleLoaderState := modload.NewState()
  add runUse://+0 moduleLoaderState := modload.NewState()
  add runVendor://+0 moduleLoaderState := modload.NewState()
  rm sync.go:/var moduleLoaderState \*modload.State/
'

Change-Id: Iadc4ffd19d15f80a694285c86adfc01f0d26bac7
Reviewed-on: https://go-review.googlesource.com/c/go/+/711129
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>