flua: move modules source into the main source directory

Follow the path of what is done with bsnmp, build the modules along
with the main binary, this allows to build the modules at a moment
where all needed libraries are already built and available in the
linker path instead of having to declare all the libraries which a
flua module will be linked to in _prebuild_libs.

Discused with: markj
Reviewed by: markj, jrtc27, kevans, imp
Accepted by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D46610

(cherry picked from commit 7899f917b1c0ea178f1d2be0cfb452086d079d23)