Consider namespaces for plugins: download-jbr -> de.itemis.mps.download-jbr and so on

[vlsi]

The current plugin names (download-jbr, generate-models, modelcheck) look unexpected, and it is hard to relate the plugin name to the website.

I guess adding de.itemis.mps to the plugin ids would make it better:

plugins {
  id("de.itemis.mps.download-jbr")
  id("de.itemis.mps.generate-models")
  id("de.itemis.mps.modelcheck")
}