HPCC-8683 Problems with PLUGIN attribute on a service definition

Fix a spurious warning when using PLUGIN attribute, and ensure that the
default plugins location is on the libpath for linking to make life easier.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4049 from richardkchapman/plugin-resolve

HPCC-8683 Problems with PLUGIN attribute on a service definition

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>