support monorepos / repository.directory

[tunnckoCore]

Currently, to support that we need to pass a full location link to package (e.g. https://github.com/user/repo/packages/foobar to the repository field, which isn't recommended in general.

There's also another reason for me to use the repository field that way, and it's because npm's package page doesn't respect repository.directory too (the "repository" link).

The only left possible workaround is to put it on the homepage but if you want to have a website there it's not an option.

For this purpose, we can instead use package-json or such package, to get the current package.json for a package.