Move repo:purge_cache to heroku-builds

[edmorley]

Hi!

As discussed in heroku/cli#509 and heroku/heroku-repo#69, it seems like the heroku-repo plugin isn't the best fit for the cache related command repo:purge_cache, particularly now that the caches are no longer tied to the git repo.

As such, heroku-builds seems like a good home, perhaps in the form of builds:purge-cache or heroku builds:cache:purge.

The existing code is here:
https://github.com/heroku/heroku-repo/blob/3e7167d79ca06f8be9cc4b084f7ca03d5adaee46/commands/purge_cache.js
https://github.com/heroku/heroku-repo/blob/3e7167d79ca06f8be9cc4b084f7ca03d5adaee46/lib/repo.js#L15-L16

Note there are some open tickets against this feature:

• repo:purge_cache doesn't purge all files heroku-repo#70: repo:purge_cache doesn't purge all files
• Incorrect error with non-existent app name heroku-repo#61: Incorrect error with non-existent app name
• Error: stdin: not in gzip format heroku-repo#57: Error: stdin: not in gzip format
• repo:purge_cache when there is nothing to purge heroku-repo#36: repo:purge_cache when there is nothing to purge

Plus heroku/cli#509 (comment) identified the following cleanups:

• rename command to something better
• show example of usage including output
• description is unclear and has improper grammar
• description verb should be 'delete' not 'deletes'

After this move, I'd also like to add a command to download the cache file for debugging (similar to the slugs:download command from heroku-slugs) - so worth bearing in mind for the naming.

I also wonder whether the slugs:* commands from heroku-slugs should be moved under the "builds" topic too, once builds covers {builds, cache, ...}?

[dmathieu]

The old command is now deprecated, and will be removed someday in the future.