cmd/go/internal/modfetch: rename State to Fetcher

cmd/go/internal/modfetch: rename State to Fetcher

This change renames the type State to Fetcher to better reflect its
purpose.  The global variable ModuleFetchState is also renamed to
Fetcher_, which will continue to be gradually eliminated as with all
global state in the modfetch package.

[git-generate]
cd src/cmd/go/internal/modfetch
rf '
  mv State Fetcher
  mv ModuleFetchState Fetcher_
  mv NewState NewFetcher

  mv Fetcher.GoSumFile GoSumFile
  mv GoSumFile.s GoSumFile.f
  mv GoSumFile Fetcher.GoSumFile

  mv Fetcher.SetGoSumFile SetGoSumFile
  mv SetGoSumFile.s SetGoSumFile.f
  mv SetGoSumFile Fetcher.SetGoSumFile

  mv Fetcher.AddWorkspaceGoSumFile AddWorkspaceGoSumFile
  mv AddWorkspaceGoSumFile.s AddWorkspaceGoSumFile.f
  mv AddWorkspaceGoSumFile Fetcher.AddWorkspaceGoSumFile
'
rf '
  add NewFetcher:+0 f := new(Fetcher) \
  f.lookupCache = new(par.Cache[lookupCacheKey, Repo]) \
  f.downloadCache = new(par.ErrCache[module.Version, string]) \
  return f
'
rf 'rm NewFetcher:+5,8'
cd ../modload
rf '
  mv State.modfetchState State.fetcher
'

Change-Id: I7cb6c945ea0f1d2119e1615064f041e88c81c689
Reviewed-on: https://go-review.googlesource.com/c/go/+/721740
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>