refact(plugin): Move host loopback plugin into internal/plugin/loopback

internal/host/plugin/immutable_fields_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/hashicorp/boundary/internal/iam"
 	"github.com/hashicorp/boundary/internal/kms"
 	"github.com/hashicorp/boundary/internal/plugin/host"
+	"github.com/hashicorp/boundary/internal/plugin/loopback"
 	"github.com/hashicorp/boundary/internal/scheduler"
 	plgpb "github.com/hashicorp/boundary/sdk/pbs/plugin"
 	"github.com/stretchr/testify/assert"
@@ -108,7 +109,7 @@ func TestPluginSet_ImmutableFields(t *testing.T) {
 	plg := host.TestPlugin(t, conn, "test")
 	cat := TestCatalog(t, conn, prj.PublicId, plg.GetPublicId())
 	new := TestSet(t, conn, kmsCache, sched, cat, map[string]plgpb.HostPluginServiceClient{
-		plg.GetPublicId(): NewWrappingPluginClient(&TestPluginServer{}),
+		plg.GetPublicId(): loopback.NewWrappingPluginClient(&loopback.TestPluginServer{}),
 	})
 
 	tests := []struct {