Add support for bundled private composer dependencies via path repo

[felixbrucker]

Currently with the upcoming changes in Shopware v6.4.0.0 which will allow to mark a plugin as composer plugin and thus let composer manage its dependencies (https://github.com/shopware/platform/commit/59d90d70f177e204ba606905a80417f60ad5e515), there is a need to also support private dependencies which you bundle with the plugin. This PR enables the usage of private dependencies through a path repo packages within each plugin directory.

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[felixbrucker]

Wrong repo 🤦