add symlink link: dependency type (fixes #884)

[mgcrea]

Summary

Implements the link: dependency type, see #884 for reasons.

I tried to integrate it as smoothly as possible in the existing code base, I chose to implement a noop LinkFetcher class instead of forking away the default resolving/fetching process.

Will gladly refactor if needed.

Test plan

With the following package.json:

{
  "name": "@mgcrea/test-yarn-link",
  "version": "1.0.0",
  "dependencies": {
    "@mgcrea/core-users": "link:./../core-users",
    "express-restify": "link:./../express-restify"
  },
  "devDependencies": {}
}

$ rm -rf node_modules; yarn
yarn install v0.15.1
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 📃  Building fresh packages...
✨  Done in 0.14s.

$ ll node_modules/
total 4.0K
drwxr-xr-x 3 olivier staff 102 Oct 16 00:46 '@mgcrea'/
lrwxr-xr-x 1 olivier staff  21 Oct 16 00:46 express-restify -> ../../express-restify/

Had to improve the possibleExtraneous logic to handle scoped packages as well (scope folder would be removed on yarn add).

[sebmck]

Going to close this until we've got an accepted RFC. Please see the yarnpkg/rfcs repo for more information.

[benlangfeld]

@kittens Could we re-open this now that the RFC is accepted in principle and npm 5 is implementing the same feature?

[arcanis]

Github doesn't let me reopen it :( @mgcrea, can you try to start a new PR?