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/mem/mem_backend.go

@@ -71,7 +71,7 @@ func (be *MemoryBackend) Test(ctx context.Context, h restic.Handle) (bool, error
 
 // IsNotExist returns true if the file does not exist.
 func (be *MemoryBackend) IsNotExist(err error) bool {
-	return errors.Cause(err) == errNotFound
+	return errors.Is(err, errNotFound)
 }
 
 // Save adds new Data to the backend.