improvement behavior of files field in package.json

[pastak]

Summary

This pr make improve behavior of files in package.json.

According to npm docs, files field's priority is higher than some ignoring (ex: .npmignore, .gitignore) and foo.js will be treated as both foo.js and foo.js/**.

On example project like below

.
├── README.md
├── dist
│   ├── command.js
│   ├── index.js
│   └── libs
│       └── util.js
├── package.json
├── .gitignore
├── node_modules
│   └── ....
├── src
│   ├── command.js
│   ├── index.js
│   └── libs
│       └── util.js
└── yarn.lock

and .gitignore is

dist

with package.json has {"files": ["dist"]}

Result of npm pack is

$ npm pack
example-0.1.0.tgz
$ tar -tzf example-0.1.0.tgz
package/package.json
package/README.md
package/dist/command.js
package/dist/index.js
package/dist/libs/util.js

But yarn pack is

$ yarn pack
yarn pack v0.23.2
success Wrote tarball to "/Users/pastak/works/example/example-v0.1.0.tgz".
$ tar -tzf example-v0.1.0.tgz
package
package/LICENSE
package/README.md
package/dist
package/package.json

Because of that, yarn publish will publish package with empty dist directory.

Test plan

$ cd /PATH/TO/THIS_YARN_REPO/
$ yarn build
$ cd /PATH/TO/EXMPLE_PACKAGE_DIR
$ /PATH/TO/THIS_YARN_REPO/bin/yarn pack
yarn pack v0.23.2
success Wrote tarball to "/Users/pastak/works/example/example-v0.1.0.tgz".
$ tar -tzf example-v0.1.0.tgz
package
package/LICENSE
package/README.md
package/dist/command.js
package/dist/index.js
package/dist/libs/util.js
package/package.json

[bestander]

Looks like fixed in #3175