box: fix performance problem reading metadata for single files

Before this change the backend used to list the directory to find the
metadata for a single file. For lots of files in a directory this
caused a serious performance problem.

This change uses the preflight check to check for a files existence
and find its ID.

See: https://forum.rclone.org/t/psa-box-com-has-serious-performance-issues-in-directories-with-thousands-of-files/41128/10
See: https://forum.box.com/t/is-there-an-api-to-find-a-file-by-leaf-name-given-a-folder-id/997/
See: https://developer.box.com/guides/uploads/check/

backend/box/api/types.go

@@ -167,19 +167,7 @@ type PreUploadCheckResponse struct {
 // PreUploadCheckConflict is returned in the ContextInfo error field
 // from PreUploadCheck when the error code is "item_name_in_use"
 type PreUploadCheckConflict struct {
-	Conflicts struct {
-		Type        string `json:"type"`
-		ID          string `json:"id"`
-		FileVersion struct {
-			Type string `json:"type"`
-			ID   string `json:"id"`
-			Sha1 string `json:"sha1"`
-		} `json:"file_version"`
-		SequenceID string `json:"sequence_id"`
-		Etag       string `json:"etag"`
-		Sha1       string `json:"sha1"`
-		Name       string `json:"name"`
-	} `json:"conflicts"`
+	Conflicts ItemMini `json:"conflicts"`
 }
 
 // UpdateFileModTime is used in Update File Info