Anchor package.json files to the package’s root

[tusbar]

Summary

npm pack bases paths in the files array from the package’s root.
This updates yarn pack to behave in the same way.

Test plan

Let’s consider the following package:

{
  "name": "files-include-from-root",
  "version": "1.0.0",
  "license": "MIT",
  "files": ["index.js"]
}

With the following files:

$ tree ./files-include-from-root
├── index.js
├── package.json
└── sub
    └── index.js

npm pack’s current behavior:

$ npm pack --dry-run ./files-include-from-root
npm notice
npm notice 📦  files-include-from-root@1.0.0
npm notice === Tarball Contents ===
npm notice 107B package.json
npm notice 25B  index.js
npm notice === Tarball Details ===
npm notice name:          files-include-from-root
npm notice version:       1.0.0
npm notice filename:      files-include-from-root-1.0.0.tgz
npm notice package size:  237 B
npm notice unpacked size: 132 B
npm notice shasum:        e4f148d35450d9183afd42c54625fc6ae51ea2e7
npm notice integrity:     sha512-vunAN8bueT2bi[...]UsDOZrX48L8hA==
npm notice total files:   2
npm notice
files-include-from-root-1.0.0.tgz

yarn pack’s current behavior:

$ tree ./files-include-from-root/package
./files-include-from-root/package
├── index.js
├── package.json
└── sub
    └── index.js

yarn pack with this patch:

$ tree ./files-include-from-root/package
./files-include-from-root/package
├── index.js
└── package.json

Fix #5779.

[arcanis]

Thanks @tusbar! Will be part of the next stable minor 🙂