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/sia/sia.go

@@ -3,6 +3,7 @@ package sia
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -11,18 +12,16 @@ import (
 	"strings"
 	"time"
 
-	"github.com/rclone/rclone/fs/config"
-	"github.com/rclone/rclone/lib/encoder"
-
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/backend/sia/api"
 	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/config"
 	"github.com/rclone/rclone/fs/config/configmap"
 	"github.com/rclone/rclone/fs/config/configstruct"
 	"github.com/rclone/rclone/fs/config/obscure"
 	"github.com/rclone/rclone/fs/fserrors"
 	"github.com/rclone/rclone/fs/fshttp"
 	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/lib/encoder"
 	"github.com/rclone/rclone/lib/pacer"
 	"github.com/rclone/rclone/lib/rest"
 )
@@ -460,7 +459,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 	if opt.APIPassword != "" {
 		opt.APIPassword, err = obscure.Reveal(opt.APIPassword)
 		if err != nil {
-			return nil, errors.Wrap(err, "couldn't decrypt API password")
+			return nil, fmt.Errorf("couldn't decrypt API password: %w", err)
 		}
 		f.srv.SetUserPass("", opt.APIPassword)
 	}
@@ -474,7 +473,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		}
 		_, err := f.NewObject(ctx, remote)
 		if err != nil {
-			if errors.Cause(err) == fs.ErrorObjectNotFound || errors.Cause(err) == fs.ErrorNotAFile {
+			if errors.Is(err, fs.ErrorObjectNotFound) || errors.Is(err, fs.ErrorNotAFile) {
 				// File doesn't exist so return old f
 				f.root = root
 				return f, nil
@@ -493,7 +492,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 func errorHandler(resp *http.Response) error {
 	body, err := rest.ReadBody(resp)
 	if err != nil {
-		return errors.Wrap(err, "error when trying to read error body")
+		return fmt.Errorf("error when trying to read error body: %w", err)
 	}
 	// Decode error response
 	errResponse := new(api.Error)