Feature: Allow for concurrency

[tabrindle]

If running more than one yarn instance at a time (installing for multiple package.json in a project) installing packages, there seems to be issues with caching. I do not believe this is a bug, as I don't think this was ever intended use, but would be nice for this use case.

yarn install v0.15.1
yarn install v0.15.1
yarn install v0.15.1
[1/4] 🔍  Resolving packages...
[1/4] 🔍  Resolving packages...
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[2/4] 🚚  Fetching packages...
error https://registry.yarnpkg.com/minimatch/-/minimatch-3.0.3.tgz: ENOENT: no such file or directory, open '/Users/ME/.yarn-cache/npm-minimatch-3.0.3/.yarn-tarball.tgz'
    at Error (native)
info Visit http://yarnpkg.com/en/docs/cli/install for documentation about this command.
warning xcode > simple-plist > plist > xmlbuilder > lodash-node@2.4.1: This package is discontinued. Use lodash@^4.0.0.
[2/4] 🚚  Fetching packages...
error https://registry.yarnpkg.com/babel-generator/-/babel-generator-6.17.0.tgz: ENOENT: no such file or directory, open '/Users/ME/.yarn-cache/npm-babel-generator-6.17.0/.yarn-tarball.tgz'
    at Error (native)
info Visit http://yarnpkg.com/en/docs/cli/install for documentation about this command.
[3/4] 🔗  Linking dependencies...
[4/4] 📃  Building fresh packages...
success Saved lockfile.
✨  Done in 11.49s.

Using:
node v6.8.0
yarn v0.15.1

[yunxing]

Maybe for now, try specify "--packages-root" to an isolated directory for each instance.

[tabrindle]

That's a good suggestion - does that hinder caching/performance?

[sebmck]

Dupe of #683.