Always include npm shrinkwrap data in pack

Packages which have a `files` property in their `package.json` file and want to publish a dependency-frozen package with [npm-shrinkwrap](https://www.npmjs.org/doc/cli/npm-shrinkwrap.html) 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"],
}
```