Remove junk from dist

[Daniel15]

There's a bunch of files in dist that we don't actually need, mainly readmes and examples from third-party modules. This unnecessarily bloats out the download size. I already had to clean up some of them to avoid lint errors in the Debian package (https://github.com/yarnpkg/yarn/blob/master/scripts/build-deb.sh#L45-L47) but we should try to clean up other stuff where possible.

[Daniel15]

There's ~2.3 MB of files in node_modules\rx\ts we probably don't need

[jamiebuilds]

Here's a fun idea, you could use module-deps to find all the things that are actually necessary to run Yarn and only copy those files into dist

[Daniel15]

That's an interesting idea. We do have the standalone .js file built using Webpack, I wonder if we can get Webpack to output the dependency tree in a similar way and use that.

We do need to keep the LICENSE files, so we should be sure to special-case those regardless of the approach we take 😄

[Daniel15]

Not needed now that we have a flat bundle (#3030)