Fix related google calendars not being found (bitfireAT/davx5#409)

* Minor changes
- update kdoc
- rename method and variables

* Add proxy parents to related resource detection

* Rename argument, query ResourceType

* Remove unnecessary utility method

* Change parentOf to extension function; Always return URL with trailing slash

* Use calendar-proxy-read/write ResourceType from new dav4jvm

* Use max. two levels of recursion to detect shared Google calendars

* Revise test and adapt method

* Simplify HttpUrl.parent()

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Co-authored-by: Ricki Hirner <hirner@bitfire.at>