I have a directory structure like so:
composer.json < Main packages/ balunker/ testpackage/ composer.json < Package src/
For future Googlers, add your version to the composer.json and then require the package with the --prefer-source option.
composer.json
--prefer-source
For example: composer require your-vendor/package:1.0.* --prefer-source
composer require your-vendor/package:1.0.* --prefer-source