cmd/go: refactor usage of `initialized`

cmd/go: refactor usage of `initialized`

This commit refactors usage of the global variable `initialized` to
the global LoaderState field of the same name.

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

[git-generate]
cd src/cmd/go/internal/modload
rf 'ex { initialized -> LoaderState.initialized }'
rf 'rm initialized'

Change-Id: I97e35bab00f4c22661670b01b69425fc25efe6df
Reviewed-on: https://go-review.googlesource.com/c/go/+/698056
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>