Always include npm shrinkwrap data in pack

[mathbruyen]

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Packages which have a files property in their package.json file and want to publish a dependency-frozen package with npm-shrinkwrap need to explicitly state that npm-shrinkwrap.json file is to be included in the tarball.

NPM metadata is to be automatically included in the tarball, like package.json is.

Packages used to require an npm-specific file in their declaration:

{
  "name": "foobar",
  "version": "0.0.1",
  "files": ["build.js", "npm-shrinkwrap.json"],
}

Now just need to specify their own file:

{
  "name": "foobar",
  "version": "0.0.1",
  "files": ["build.js"],
}

[iarna]

This will need a test before we land it. fstream releated tests are hard to work with, so I'll write one at some point, but that work isn't scheduled yet. (If you were to add one, we could land this immediately.)