Do not download dependencies if not needed

[victornoel]

Do you want to request a feature or report a bug?
feature

What is the current behavior?
If I have a fully populated node_modules (with respect to the lockfile) and my cache is empty, if I call yarn --pure-lockfile, yarn will download the dependencies to the cache even though they already are in my node_modules.

What is the expected behavior?
If there is no need to install a new package to node_modules, then yarn should not download it to the cache.

Why?
It is related to #986 in a way: I want to avoid downloading packages when it is not needed.

Please mention your node.js, yarn and operating system version.
yarn v0.15.1, node v6.7.0 and linux

[bestander]

Fixed now