fix: support package.json private directive

[Nytelife26]

Prior to this, eslint-plugin-node would raise a no-unpublished-* if a file
was not published, even if the package itself was set to private. This PR fixes
that by making util/check-publish respect the private directive in
package.json.

[aladdin-add]

we are maintaining a fork in https://github.com/eslint-community/eslint-plugin-n. and it has been landed in eslint-community@5ccd0c5.