build: split main/extras into separate bundles

Makes the offline build/p2 generation a bit more straight forward

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: adjust to new build bundles

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: fix p2 dependencies

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>