Use the get() and set() methods to set source and destination on a migration.

This is necessary after changes being discussed in [#2384529: Make the class variables protected for Migration](https://www.drupal.org/node/2384529). While that issue is not committed yet, the changes in this commit will work even now, and will continue to work after the issue is committed.

Note: There might be direct getter methods but that is under discussion in that issue. However, the changes in these pull request don't rely on those methods.