HPCC-9160 Plugins that don't export pluginDefinition block get unloaded

Since we split the ECL definition out of the embedded language libraries, they
are not remaining loaded at startup (see HPCC-9068).

This caused cores in the java embedded code test.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4249 from richardkchapman/embed-plugins-unload

HPCC-9160 Plugins that don't export pluginDefinition block get unloaded

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>