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/eval_handler.go

@@ -7,18 +7,18 @@ import (
 
 	"github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/infra/metrics"
-	"github.com/grafana/grafana/pkg/plugins"
+	"github.com/grafana/grafana/pkg/tsdb/legacydata"
 )
 
 // DefaultEvalHandler is responsible for evaluating the alert rule.
 type DefaultEvalHandler struct {
 	log             log.Logger
 	alertJobTimeout time.Duration
-	requestHandler  plugins.DataRequestHandler
+	requestHandler  legacydata.RequestHandler
 }
 
 // NewEvalHandler is the `DefaultEvalHandler` constructor.
-func NewEvalHandler(requestHandler plugins.DataRequestHandler) *DefaultEvalHandler {
+func NewEvalHandler(requestHandler legacydata.RequestHandler) *DefaultEvalHandler {
 	return &DefaultEvalHandler{
 		log:             log.New("alerting.evalHandler"),
 		alertJobTimeout: time.Second * 5,