always include main file, even if not configured

[zkat]

Fixes: npm/npm#8510

[jasonkarns]

Does it make sense for this to be hardcoded? This bit of knowledge (ie, the default main filename) ideally shouldn't be duplicated anywhere. And the logic for normalizing the package.json into an object with defaults and various formattings already exists in https://github.com/npm/read-package-json

I would expect that fstream instead just read from the normalized package.json to be consistent?