Update check_emoji_sequences.

- use existing utilities in nototools/unicode_data, add_aliases
- add check that file names do not use presentation selectors
- include tags in valid cps that can appear in a sequence
- add check for valid tag sequences (for subregion flags)
- separate out check that no source for an alias is present (we
  expect to alias this so should not have an image with that name)
- filter data by age (somewhat), provide command line flag, remove
  hard-coded unicode 9.0 value
- separate coverage check (for when data is partial), provide
  command line flag and don't run by default
- provide command line flag to exclude subdirs by name when collecting
  images
- refactor output so each error has a consistent initial text indicating
  the check where the error was found, make output a tad less verbose