WIP: Use Licensed manifest method for caching license metadata

[lildude]

Floating this WIP early to ease getting feedback as I work this. I'll add in the apt parts of the template later.

For the moment, I'm working on using the latest licensed directly as I iterate on the configuration:

$ bundle exec licensed cache -c vendor/licenses/config.yml
Caching licenses for grammars:
  manifest dependencies:
License caching complete!
* grammars manifest dependencies: 0
$

Commented out parts are things I've considered and will need to reconsider later too, so feel free to ignore those parts or comment as necessary.

I'll update the script/licensed and script/add-grammar later.

/cc @jonabc #4265

[lildude]

After a fair bit of experimentation over the weekend, I'm going to defer on this idea in favour of updating our current approach. This is primarily because using the manifest requires monkey patching the current behaviour and implementing functionality necessary to automatically add a new line to the config.yml everytime a new grammar is added. This is far more complex than simply adding a version to all our current license files.

[jonabc]

This is far more complex than simply adding a version to all our current license files.

👍 let me know if theres anything else I can help with. I think you should still be able to leverage licensed to test for allowed licenses, and remove license tests from https://github.com/github/linguist/blob/8bf9efa3702a1a43df85dc5cd72b63f3ff36871f/test/test_grammars.rb. Though of course that choice is up to linguist peeps 😄