pydev is expecting to find pydev_ext file in the actual eclipse installation root. It does so even if the pydev plugins are installed in a different place and linked by file in $ECLIPSE_HOME/links.
In my case I have eclipse in '/opt/eclipse' and pydev in '/opt/eclipse-ext/pydev' but PyDev only looks into /opt/eclipse/features/com.python.pydev when searching for pydev_ext and pydev_ext_mail. It means /opt/eclipse-ext/pydev/eclipse/features/com.python.pydev is ignored.
Without this it is harder to make a simple package that will be independent from the eclipse as much as possible.