Yarn: Add `yarn cache dir` to output cache directory

[cnolimit]

When reading the blog post it stated that during the fetching phase/step, yarn looks inside the global cache directory. Which folder is this exactly?

[montogeek]

~/.yarn-cache

[ljharb]

npm offers a number of npm cache commands - could yarn offer the same?

[montogeek]

It should :)

[sindresorhus]

yarn cache ls
yarn cache clean

[ljharb]

ideally yarn cache dir or something would print out the path (npm doesn't have this, sadly)

[sebmck]

Created #880 to fix this, pending approval.

[manojlds]

Not related, but is there a way to change the yarn cache dir?

[xzyfer]

npm also expose the cache dir the script environment as process.env.npm_config_cache. Is this covered also?