Bug#952390: paraview: 3rd party plugins dir violates directory standards

[Drew Parsons]

Package: paraview
Version: 5.13.1+dfsg-1
Followup-For: Bug #952390
Control: forwarded 952390 https://gitlab.kitware.com/paraview/paraview/-/issues/21661

I think this problem with /usr/bin/plugins is a side effect of
https://gitlab.kitware.com/paraview/paraview/-/blob/master/Remoting/Core/vtkPVPluginLoader.cxx#L321
which adds /plugin in appDir = pm->GetSelfDir(), which is the
executable directory.

Before that line, other plugin paths are added, if defined in
PARAVIEW_PLUGIN_LOADER_PATHS

So I think the problem may be that we don't have
PARAVIEW_PLUGIN_LOADER_PATHS configured.
The default plugin path is supposed to be
/usr/lib/*/paraview-${paraview_version_suffix}/plugins/
as set in LIBRARY_SUBDIRECTORY in toplevel CMakeLists.txt
and PARAVIEW_PLUGIN_SUBDIR in CMake/paraview-config.cmake.in
see also CMake/ParaViewPlugin.cmake

/usr/lib/*/paraview-${paraview_version_suffix}/plugins/
is not currently showing in the PluginManager.
If it can be listed there, then that may not directly solve the
problem of "/usr/bin/plugins", but at least it won't leave us thinking
it is the "correct" plugin path.

cf. https://gitlab.kitware.com/paraview/paraview/-/issues/21661#note_1583135

It might be that we need to activate configuration of
PARAVIEW_PLUGIN_LOADER_PATHS for the debian build.