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/tsdb/intervalv2/intervalv2.go

@@ -8,11 +8,10 @@ import (
 
 	"github.com/grafana/grafana-plugin-sdk-go/backend"
 	"github.com/grafana/grafana-plugin-sdk-go/backend/gtime"
-	"github.com/grafana/grafana/pkg/tsdb/interval"
 )
 
 var (
-	defaultRes         int64 = 1500
+	DefaultRes         int64 = 1500
 	defaultMinInterval       = time.Millisecond * 1
 	year                     = time.Hour * 24 * 365
 	day                      = time.Hour * 24
@@ -59,18 +58,18 @@ func (ic *intervalCalculator) Calculate(timerange backend.TimeRange, minInterval
 	from := timerange.From.UnixNano()
 	resolution := maxDataPoints
 	if resolution == 0 {
-		resolution = defaultRes
+		resolution = DefaultRes
 	}
 
 	calculatedInterval := time.Duration((to - from) / resolution)
 
 	if calculatedInterval < minInterval {
-		return Interval{Text: interval.FormatDuration(minInterval), Value: minInterval}
+		return Interval{Text: FormatDuration(minInterval), Value: minInterval}
 	}
 
 	rounded := roundInterval(calculatedInterval)
 
-	return Interval{Text: interval.FormatDuration(rounded), Value: rounded}
+	return Interval{Text: FormatDuration(rounded), Value: rounded}
 }
 
 func (ic *intervalCalculator) CalculateSafeInterval(timerange backend.TimeRange, safeRes int64) Interval {
@@ -79,7 +78,7 @@ func (ic *intervalCalculator) CalculateSafeInterval(timerange backend.TimeRange,
 	safeInterval := time.Duration((to - from) / safeRes)
 
 	rounded := roundInterval(safeInterval)
-	return Interval{Text: interval.FormatDuration(rounded), Value: rounded}
+	return Interval{Text: FormatDuration(rounded), Value: rounded}
 }
 
 // GetIntervalFrom returns the minimum interval.