chore(ur): move structs to reduce dependency chain

lib/ur brings in a lot of dependencies we don't need in e.g.
stcrashreceiver, who only needs the small failure reporting structs.
Make those part of the lean `contract` package instead.

Signed-off-by: Jakob Borg <jakob@kastelo.net>

cmd/infra/stcrashreceiver/main.go

@@ -30,7 +30,7 @@ import (
 	raven "github.com/getsentry/raven-go"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/syncthing/syncthing/lib/build"
-	"github.com/syncthing/syncthing/lib/ur"
+	"github.com/syncthing/syncthing/lib/ur/contract"
 )
 
 const maxRequestSize = 1 << 20 // 1 MiB
@@ -131,7 +131,7 @@ func handleFailureFn(dsn, failureDir string, ignore *ignorePatterns) func(w http
 			return
 		}
 
-		var reports []ur.FailureReport
+		var reports []contract.FailureReport
 		err = json.Unmarshal(bs, &reports)
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusBadRequest)
@@ -179,7 +179,7 @@ func handleFailureFn(dsn, failureDir string, ignore *ignorePatterns) func(w http
 	}
 }
 
-func saveFailureWithGoroutines(data ur.FailureData, failureDir string) (string, error) {
+func saveFailureWithGoroutines(data contract.FailureData, failureDir string) (string, error) {
 	bs := make([]byte, len(data.Description)+len(data.Goroutines))
 	copy(bs, data.Description)
 	copy(bs[len(data.Description):], data.Goroutines)