cmd/go: convert functions to methods

cmd/go: convert functions to methods

This commit converts the Reset and setState functions to methods.

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

[git-generate]
cd src/cmd/go/internal/modload
rf '
  mv setState State.setState
  mv Reset State.Reset
'

Change-Id: Ibc40071dc044ef5d1ab0a0b03f17b75243a42011
Reviewed-on: https://go-review.googlesource.com/c/go/+/712702
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>