Replace getMainClassFromJar with getMainClassFromPlugin

[loosebazooka]

This will allow us to customize main class searching in the plugins.

For example also searching in application plugin in gradle

There's probably a little more involved here: getJarPluginName might need to be reworked.

[loosebazooka]

Maybe return a map?

{
"jar task": "some.main.Class"
"application plugin": "some.main.Class"
}

or do all the processing on the plugin side, so we can error out if they define it twice or some other logic to handle conflicts.