Remove github.com/pkg/errors and replace with std library version

This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.

This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.

backend/hasher/commands.go

@@ -2,9 +2,10 @@ package hasher
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"path"
 
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/cache"
@@ -118,18 +119,18 @@ func (f *Fs) dbImport(ctx context.Context, hashName, sumRemote string, sticky bo
 	case fs.ErrorIsFile:
 		// ok
 	case nil:
-		return errors.Errorf("not a file: %s", sumRemote)
+		return fmt.Errorf("not a file: %s", sumRemote)
 	default:
 		return err
 	}
 
 	sumObj, err := sumFs.NewObject(ctx, path.Base(sumPath))
 	if err != nil {
-		return errors.Wrap(err, "cannot open sum file")
+		return fmt.Errorf("cannot open sum file: %w", err)
 	}
 	hashes, err := operations.ParseSumFile(ctx, sumObj)
 	if err != nil {
-		return errors.Wrap(err, "failed to parse sum file")
+		return fmt.Errorf("failed to parse sum file: %w", err)
 	}
 
 	if sticky {