Chore: Refactor usage of legacy data contracts (#41218)

Refactor usage of legacy data contracts. Moves legacy data contracts 
to pkg/tsdb/legacydata package.
Refactor pkg/expr to be a proper service/dependency that can be provided 
to wire to remove some unneeded dependencies to SSE in ngalert and other places.
Refactor pkg/expr to not use the legacydata,RequestHandler and use 
backend.QueryDataHandler instead.

pkg/services/alerting/engine.go

@@ -11,10 +11,10 @@ import (
 	"github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/infra/usagestats"
 	"github.com/grafana/grafana/pkg/models"
-	"github.com/grafana/grafana/pkg/plugins"
 	"github.com/grafana/grafana/pkg/services/encryption"
 	"github.com/grafana/grafana/pkg/services/rendering"
 	"github.com/grafana/grafana/pkg/setting"
+	"github.com/grafana/grafana/pkg/tsdb/legacydata"
 	"github.com/opentracing/opentracing-go"
 	"github.com/opentracing/opentracing-go/ext"
 	tlog "github.com/opentracing/opentracing-go/log"
@@ -28,7 +28,7 @@ type AlertEngine struct {
 	RenderService    rendering.Service
 	Bus              bus.Bus
 	RequestValidator models.PluginRequestValidator
-	DataService      plugins.DataRequestHandler
+	DataService      legacydata.RequestHandler
 	Cfg              *setting.Cfg
 
 	execQueue         chan *Job
@@ -48,7 +48,7 @@ func (e *AlertEngine) IsDisabled() bool {
 
 // ProvideAlertEngine returns a new AlertEngine.
 func ProvideAlertEngine(renderer rendering.Service, bus bus.Bus, requestValidator models.PluginRequestValidator,
-	dataService plugins.DataRequestHandler, usageStatsService usagestats.Service, encryptionService encryption.Service,
+	dataService legacydata.RequestHandler, usageStatsService usagestats.Service, encryptionService encryption.Service,
 	cfg *setting.Cfg) *AlertEngine {
 	e := &AlertEngine{
 		Cfg:               cfg,