Respect relativePaths in local package paths (#1498)

* Respect relativePaths in local package paths

Before this a local package defined in an included file would not
respect relativePaths: true.

This fixes #1497.

* Test local package with relative path

* add relative paths support to localPackages syntax

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Co-authored-by: Yonas Kolb <yonaskolb@users.noreply.github.com>