bower was calling `__fish_should_complete_args`, the old name for
`__fish_should_complete_switches.`
yarn was parsing bower.json instead of package.json.
Now parses package.json and uses results to provide a list of possible
completions to `yarn remove`. There may be other subcommands that could
benefit from this.
Could have parsed yarn output, but yarn is slow and packages.json format
is generally standard since it's machine-generated json.