define the format of the gradle/maven fields: artifact_id, group_id, gradle, and maven_repository

For F-Droid, the fields _artifact_id_, _group_id_, _gradle_, and _maven_repository_ are the most useful. But I'm not clear what their format is supposed to be. Are they defined as a single string entry? Or regexps? Or some a string others regexps? Here's my gut feeling about what they should be:

* _maven_repository_ should be a list of URL strings, each one being fully formed and directly usable
* _group_id_ should be a list of regexps to match Maven group identifiers
* _artifact_id_ should be a list of regexps to match Maven artifact identifiers
* _gradle_ I'm not sure what this is in relation to _group_id_ and _artifact_id_, since it contains both. This could be a list of complete strings that can be used in combination with _maven_repository_ to download example versions. It should include an entry for each known SDK artifact with a known working version. This can then be used for matching in _build.gradle_ by stripping the version and maybe artifact identifer.

These fields are different than the code/network signatures since they can be used to both match declarations in _build.gradle_ and also to download the actual artifacts. If they are regexps, then they cannot be used to download, only to match.