When opening any geotools-modul from plugin-dependencies in eclipse, src-link does not function and only class-file is displayed.
Solution or much more a workaround:
after copying all src-balls from
udig-1.2-snapshot/sdk/plugins/net.refractions.udig_platform.source_126.96.36.199007121358/src/net.refractions.udig.libs_188.8.131.52007121358/lib and restart/reload target platform,
sources are linked to class-files again. I don't know how to fix it in udig-build-system, but for me this workaround works and debugging is not such a big pain.