Revisit naming issue for ErrorProneChecksExtension and ErrorProneChecksPlugin

[alexander-yevsyukov]

It would be better to hide the implementation details (e.g. ErrorProne) in the naming. Also, the extension and plugin look a bit rudimentary, especially assuming a separate class SeverityConfigurer and the presence of defaultChecksSeverity property of the io.spine.tools.mc.java.gradle.Extension.

It would be probably better to have all the model check settings under the modelCompiler like this:

modelCompiler {
     checks {
           defaultSeverity = ERROR;
           useValidatingBuildersSeverity = WARNING;
     }
    ...
}

Please also see this PR comment for more details.