Module plugins are (still) built in <build-dir>/modules.
When installing, f00bar binary is installed to <install-dir>/bin, and
the module plugins to <install-dir>/lib/f00bar.
For this to work, we now also set RPATH correctly. Since the installed
module plugins end up in a different location then when building,
different settings is required for BUILD_RPATH and INSTALL_RPATH.
TODO: optimizations and proper cleanup
* We currently reload the shared library for each *instance* of the
module, and we do it twice; once when verifying, and once when
instantiating.
* The shared libraries are never dlclosed()
