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.

fs/config/configstruct/configstruct.go

@@ -2,12 +2,12 @@
 package configstruct
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 	"regexp"
 	"strings"
 
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/fs/config/configmap"
 )
 
@@ -35,7 +35,7 @@ func StringToInterface(def interface{}, in string) (newValue interface{}, err er
 	o := reflect.New(typ)
 	n, err := fmt.Sscanln(in, o.Interface())
 	if err != nil {
-		return newValue, errors.Wrapf(err, "parsing %q as %T failed", in, def)
+		return newValue, fmt.Errorf("parsing %q as %T failed: %w", in, def, err)
 	}
 	if n != 1 {
 		return newValue, errors.New("no items parsed")
@@ -115,7 +115,7 @@ func Set(config configmap.Getter, opt interface{}) (err error) {
 				// it isn't valid for all types.  This makes
 				// empty string be the equivalent of unset.
 				if configValue != "" {
-					return errors.Wrapf(err, "couldn't parse config item %q = %q as %T", defaultItem.Name, configValue, defaultItem.Value)
+					return fmt.Errorf("couldn't parse config item %q = %q as %T: %w", defaultItem.Name, configValue, defaultItem.Value, err)
 				}
 			} else {
 				newValue = newNewValue