Support suppression of dependency checking

- Add a syntax (+packagename) to force inclusion as a dependency,
  even if it's not present in the APT cache.  This is probably a bad
  idea, but we need it for, e.g., things in our repository, since
  chdist can't support multiple sources.  To minimize potential
  damage, this syntax is only allowed in distro-specific list files.
Force inclusion of openfoam packages

- Since dependency-checking can't be used for packages not in upstream
  repositories, force them to be included.