cmd/go: move functions to methods

cmd/go: move functions to methods

[git-generate]
cd src/cmd/go/internal/modload
rf '
  mv InitWorkfile State.InitWorkfile
  mv FindGoWork State.FindGoWork
  mv WillBeEnabled State.WillBeEnabled
  mv Enabled State.Enabled
  mv inWorkspaceMode State.inWorkspaceMode
  mv HasModRoot State.HasModRoot
  mv MustHaveModRoot State.MustHaveModRoot
  mv ModFilePath State.ModFilePath
'

Change-Id: I207113868af037c9c0049f4207c3d3b4c19468bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/716602
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>