Don't fail on non-existent plugin directory.

[JulianGro]

When no plugins are available, Qt doesn't create a folder for them. For example, Qt might build with printsupport, but not create the printsupport plugins directory, because CUPS isn't installed and no libcupsprintersupport.so plugin was built.

Without this patch, packaging fails for me when trying to package Qt provided by Conan:

[qt/stdout] -- Deploying module: positioning -- 
[qt/stdout] Deploying Qt position plugins 
[qt/stdout] Deploying shared library /home/juliangro/.conan2/p/b/qtd40e1b81d798c/p/plugins/position/libqtposition_positionpoll.so (destination: /home/juliangro/git/overte/build/_CPack_Packages/Linux/External/Overte-Dev-2026-02-26-0/usr/plugins/position/)
[qt/stdout] WARNING: Could not find copyright files for file /home/juliangro/.conan2/p/b/qtd40e1b81d798c/p/plugins/position/libqtposition_positionpoll.so using dpkg-query 
[qt/stdout] Deploying dependencies for ELF file /home/juliangro/.conan2/p/b/qtd40e1b81d798c/p/plugins/position/libqtposition_positionpoll.so 
[qt/stdout] Deploying shared library /home/juliangro/.conan2/p/b/qtd40e1b81d798c/p/lib/libQt5Positioning.so.5
[qt/stdout] WARNING: Could not find copyright files for file /home/juliangro/.conan2/p/b/qtd40e1b81d798c/p/lib/libQt5Positioning.so.5 using dpkg-query 
[qt/stderr] terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error'
[qt/stderr]   what():  filesystem error: directory iterator cannot open directory: No such file or directory [/home/juliangro/.conan2/p/b/qtd40e1b81d798c/p/plugins/printsupport]
[qt/stdout] 
[qt/stdout] -- Deploying module: printsupport -- 
[qt/stdout] Deploying Qt printsupport plugins 
ERROR: Failed to run plugin: qt (exit code: 6) 

Granted, this is probably an edge case, since system packages would have the plugins and custom built packages would usually find system CUPS and build the plugin.

[bjorn]

Change looks fine to me, it's good to print warnings regarding this because people may not be aware their Qt could be missing some things (though, this will only get caught when all plugins of a certain type are missing), yet it may also be intentional.

I just wonder if it would be better to report the full missing path rather than just the pluginName?

[TheAssassin]

I like that idea, but then again, I'd rather merge this now. @bjorn maybe you could send a fix or open a PR?

[TheAssassin]

Thanks @JulianGro!