backend: move backend implementation helpers to util package

This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.

internal/backend/util/errdot_119.go

@@ -0,0 +1,20 @@
+//go:build go1.19
+// +build go1.19
+
+// This file provides a function to check whether an error from cmd.Start() is
+//  exec.ErrDot which was introduced in Go 1.19.
+// This function is needed so that we can perform this check only for Go 1.19 and
+//  up, whereas for older versions we use a dummy/stub in the file errdot_old.go.
+// Once the minimum Go version restic supports is 1.19, remove this file and
+//  replace any calls to it with the corresponding code as per below.
+
+package util
+
+import (
+	"errors"
+	"os/exec"
+)
+
+func IsErrDot(err error) bool {
+	return errors.Is(err, exec.ErrDot)
+}