all: Move away from pkg/errors, easy cases

github.com/pkg/errors is no longer getting updates, because Go 1.13
went with the more flexible errors.{As,Is} function. Use those instead:
errors from pkg/errors already support the Unwrap interface used by 1.13
error handling. Also:

* check for io.EOF with a straight ==. That value should not be wrapped,
  and the chunker (whose error is checked in the cases changed) does not
  wrap it.
* Give custom Error methods pointer receivers, so there's no ambiguity
  when type-switching since the value type will no longer implement error.
* Make restic.ErrAlreadyLocked private, and rename it to
  alreadyLockedError to match the stdlib convention that error type
  names end in Error.
* Same with rest.ErrIsNotExist => rest.notExistError.
* Make s3.Backend.IsAccessDenied a private function.

internal/backend/layout.go

@@ -152,7 +152,7 @@ func ParseLayout(ctx context.Context, repo Filesystem, layout, defaultLayout, pa
 		l, err = DetectLayout(ctx, repo, path)
 
 		// use the default layout if auto detection failed
-		if errors.Cause(err) == ErrLayoutDetectionFailed && defaultLayout != "" {
+		if errors.Is(err, ErrLayoutDetectionFailed) && defaultLayout != "" {
 			debug.Log("error: %v, use default layout %v", err, defaultLayout)
 			return ParseLayout(ctx, repo, defaultLayout, "", path)
 		}