Add conventions

[jjohannes]

The plugin should be usable without configuring anything. Users may then reconfigure things if needed.

Add the following convention:

• For each Test Suite, check if <source-set-path>/java/module-info.java exists
• If it does exists: automatically configure as blackbox
• If it does not exists: automatically configure as whitebox*

*For whitebox, there will be no "requires" added automatically although there will be a need for it – e.g. requires("junit"). With, gradlex-org/java-module-dependencies#31 there will be a DSL to define these and then this plugin can integrate with that.