SPDX: Improve SPDX plugin framework
With the addition of SPDX generation in ParaView, we can now create .spdx files for each modules.
However, there is no way to generate a .spdx file for a plugin. Internal module in the plugin are covered but some plugin may not have any, and some plugin may have Qt files with specific license which would not be part of a module.
This should be improved.