fix: do not crash rule no-unpublished-require

[fasttime]

Fixes #48

This PR ensures that the rule no-unpublished-require does not crash when the package root directory is required.

This is done by making sure that npmignore.match is never called if the argument is an empty string, which is the relative path of the root directory. That's because calling npmignore.match("") results in an underlying call to ignore().createFilter()(""), where ignore fails with a TypeError.

This works under the assumption that a package can never npmignore its root directory. This does not add logic to check that the "main" file defined in package.json (or fallback file index.js) exists or is published.

I've wrapped the changed code into function calls to make it better readable, and also to avoid running isDevPackage() unnecessarily when isPrivateFile() returns truthy.

[aladdin-add]

LGTM, thanks!