Fix pack includes files from node_modules, closes #2047

[torifat]

Summary

• Only look for mandetory files (package.json, readme, license, etc) in root
• Stop fs.walk inside ignored dirs (node_modules, .git, .svn, etc), makes pack way faster

Test plan

With only webpack dependency:

before

 ❯ yarn pack
Alias tip: y pack
yarn pack v0.17.8
warning package.json: No license field
success Wrote tarball to "/Users/rifat/Developments/test/yarn/yarn-test-v1.0.0.tgz".
✨  Done in 12.54s.

after

 ❯ yarn-dev pack
Alias tip: yd pack
yarn pack v0.18.0-0
warning package.json: No license field
success Wrote tarball to "/Users/rifat/Developments/test/yarn/yarn-test-v1.0.0.tgz".
✨  Done in 0.08s.

Fixes #1114, #2047

[bestander]

@torifat, any idea why the pack test failed on Travis but not on others?

[torifat]

@bestander Same as this one: https://github.com/yarnpkg/yarn/blob/master/__tests__/commands/pack.js#L104

I will try to figure it out.

[torifat]

@bestander I have figured it out.

[SimenB]

Sneaking in a thank you to @torifat for introducing me to https://github.com/djui/alias-tips, now I can finally learn more of oh-my-zsh's git aliases :D