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/putio/fs.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -13,7 +14,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/pkg/errors"
 	"github.com/putdotio/go-putio/putio"
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/config/configmap"
@@ -80,7 +80,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (f fs.Fs,
 	httpClient := fshttp.NewClient(ctx)
 	oAuthClient, _, err := oauthutil.NewClientWithBaseClient(ctx, name, m, putioConfig, httpClient)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to configure putio")
+		return nil, fmt.Errorf("failed to configure putio: %w", err)
 	}
 	p := &Fs{
 		name:        name,
@@ -469,7 +469,7 @@ func (f *Fs) purgeCheck(ctx context.Context, dir string, check bool) (err error)
 	// check directory exists
 	directoryID, err := f.dirCache.FindDir(ctx, dir, false)
 	if err != nil {
-		return errors.Wrap(err, "Rmdir")
+		return fmt.Errorf("Rmdir: %w", err)
 	}
 	dirID := atoi(directoryID)
 
@@ -482,7 +482,7 @@ func (f *Fs) purgeCheck(ctx context.Context, dir string, check bool) (err error)
 			return shouldRetry(ctx, err)
 		})
 		if err != nil {
-			return errors.Wrap(err, "Rmdir")
+			return fmt.Errorf("Rmdir: %w", err)
 		}
 		if len(children) != 0 {
 			return errors.New("directory not empty")
@@ -647,7 +647,7 @@ func (f *Fs) About(ctx context.Context) (usage *fs.Usage, err error) {
 		return shouldRetry(ctx, err)
 	})
 	if err != nil {
-		return nil, errors.Wrap(err, "about failed")
+		return nil, fmt.Errorf("about failed: %w", err)
 	}
 	return &fs.Usage{
 		Total: fs.NewUsageValue(ai.Disk.Size),  // quota of bytes that can be used