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.

librclone/librclone/librclone.go

@@ -15,7 +15,6 @@ import (
 	"runtime/debug"
 	"strings"
 
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/config/configfile"
@@ -91,23 +90,23 @@ func RPC(method string, input string) (output string, status int) {
 	// create a buffer to capture the output
 	err := json.NewDecoder(strings.NewReader(input)).Decode(&in)
 	if err != nil {
-		return writeError(method, in, errors.Wrap(err, "failed to read input JSON"), http.StatusBadRequest)
+		return writeError(method, in, fmt.Errorf("failed to read input JSON: %w", err), http.StatusBadRequest)
 	}
 
 	// Find the call
 	call := rc.Calls.Get(method)
 	if call == nil {
-		return writeError(method, in, errors.Errorf("couldn't find method %q", method), http.StatusNotFound)
+		return writeError(method, in, fmt.Errorf("couldn't find method %q", method), http.StatusNotFound)
 	}
 
 	// TODO: handle these cases
 	if call.NeedsRequest {
-		return writeError(method, in, errors.Errorf("method %q needs request, not supported", method), http.StatusNotFound)
+		return writeError(method, in, fmt.Errorf("method %q needs request, not supported", method), http.StatusNotFound)
 		// Add the request to RC
 		//in["_request"] = r
 	}
 	if call.NeedsResponse {
-		return writeError(method, in, errors.Errorf("method %q need response, not supported", method), http.StatusNotFound)
+		return writeError(method, in, fmt.Errorf("method %q need response, not supported", method), http.StatusNotFound)
 		//in["_response"] = w
 	}