Fixed java build escalation caused by generated folder cleanup

The following sequence led to escalated java build and removed
unrelated files out target/classes folders (maven plugin.xml
for example)

- Full build starts
- During 'clean' part of the full build, modello build participant
  deletes target/generated-sources/modello folder
- During 'build' part of the full build, JavaBuilder ignores
  optional target/generated-sources/modello directory, that is
  behaves as if this directory was not part of the project
  classpath
- During 'build' part of the full build, modello build paricipant
  generates target/generated-sources/modello folder
- Secondary incremental build starts
- JavaBuilder detects target/generated-sources/modello was created
  and treats this as classpath change, which results in build
  escalation to full build again.

The fix is to keep target/generated-sources/modello folder itself
but delete all its members during modello build participant
'clean'.

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
pom.xml cleanup

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>