cmd/go/internal/modload: make State.modfetchState a pointer

cmd/go/internal/modload: make State.modfetchState a pointer

This change aligns modfetch.State with modload.State by using pointer
parameters and receivers.

[git-generate]
cd src/cmd/go/internal/modload
sed -i '
  s/oldState/old/
  s/old := State{/old = \&State{/
  s/func (s \*State) setState(new State) State {/func (s *State) setState(new *State) (old *State) {/
  s/setState(State{})/setState(NewState())/
' init.go
cd ../modfetch
sed -i '
  s/oldState = State{/oldState = \&State{/
  s/func SetState(newState State) (oldState State) {/func SetState(newState *State) (oldState *State) {/
  s/SetState(State{})/SetState(NewState())/
' fetch.go
cd ../modload
sed -i '
  s/old.modfetchState = modfetch.SetState(new.modfetchState)/_ = modfetch.SetState(\&new.modfetchState)/
' init.go
rf '
  #
  # Prepare to swap the existing modfetchState field for a pointer type
  #
  mv State.modfetchState State.modfetchState_
  add State:/modfetchState_/+0 modfetchState *modfetch.State
  #
  # Update State.setState to set & restore additional values
  #
  add State.setState:/s\.requirements,/+0 workFilePath: s.workFilePath,
  add State.setState:/s\.workFilePath,/+0 modfetchState: s.modfetchState,
  #
  # Swap the existing modfetchState field for a pointer type
  #
  add init.go:/.* = modfetch.SetState\(.*\)/-0 s.modfetchState = new.modfetchState
  add init.go:/.* = modfetch.SetState\(.*\)/-0 old.modfetchState = modfetch.SetState(s.modfetchState) // TODO(jitsu): remove after completing global state elimination
  rm init.go:/_ = modfetch.SetState\(.*\)/
  rm State.modfetchState_
'
sed -i '
  s/return &State{}/s := new(State)\ns.modfetchState = modfetch.NewState()\nreturn s/
' init.go
go fmt

Change-Id: I0602ecf976fd3ee93844e77989291d729ad71595
Reviewed-on: https://go-review.googlesource.com/c/go/+/720900
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>