Merge branch 'merge'

* merge:
  Port check_dsc_files to use get_format_from_string.
  Make build_file_list to use get_format_from_string and friends.
  Add get_format_from_string utility method
  Attach validate_format(...) to each SourceFormat that can reject formats.
  Split out parse_format to module-level and test it explicitly.
  Split parsing of "Format:" string and validation of it.
  Compare None with object identity, not equality
  "Format: 0.0" is now invalid
  Add regression tests for parse_format.
  Move "Format:" field parsing into srcformats.py