Add symlink link: dependency type

[mgcrea]

We've been using some kind of monorepo approach for our projects for quite some time, a bit like lerna but with a more private and nested approach (our packages aren't expected to be published for now) along some specific needs: we must to link some public dependencies (eg. mongoose) that must be the exact same instance accross our subpackages (otherwise you encounter a lot of exotic bugs, edge cases), we also link our devDeps (they are shared accross all our subpackages).

At first we used linklocal with npm@2, leveraging a custom use of the file: prefix (basically just symlinking them), but npm@3 broke a lot of things related to their handling (eg. #10343). We ended up moving to ied where we implemented the file: prefix handling using simple symlinks, that tackled our need.

I would love to be able to switch theses project to yarn but I would need a way to create this links, I can see two ways to achieve that:

• Either add a new link: prefix that would just create symlinks and that's it
• Add an option flag --link-file-dependencies that would override default file: handling (copying) and use symlinks instead.

Would gladly submit a PR with tests if you would consider such request.

[sebmck]

Awesome idea!