cmd/go: refactor usage of `MainModules`

cmd/go: refactor usage of `MainModules`

This commit refactors usage of the global variable `MainModules` to
the global LoaderState variable 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 'mv State.mainModules State.MainModules'
rf 'ex { MainModules -> LoaderState.MainModules }'
for dir in load modcmd modget test tool workcmd ; do
  cd ../${dir}
  rf 'ex {
    import "cmd/go/internal/modload"
    modload.MainModules -> modload.LoaderState.MainModules
  }'
done
cd ../modload
rf 'rm MainModules'

Change-Id: I15644c84190717d62ae953747a288ec6495ef168
Reviewed-on: https://go-review.googlesource.com/c/go/+/698060
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>