plugins as separate packages for desktop application

[Jack Hill]

Hi Guix,

I've been thinking about https://issues.guix.gnu.org/issue/34759 again 
recently. 34759 is an issue with GIMP in which it does not find plugins 
that are installed via separate packages in the same profile. I believe 
that what is going on is that GIMP is only looking for plugins in its 
store directory and in the user's XDG_CONFIG_DIR.

I've run into a similar issue (no ticket yet) with Blender. It seems like 
this might be a problem common to many applications, and I'm wondering if 
we should have a common solution. I don't really like the symlink 
workaround I used in the GIMP ticket because it won't work in the context 
of guix environment.

I think a good way forward would be to work with upstreams to have their 
applications support an APPNAME_PLUGIN_PATH variable and when plugins are 
present in a profile they would augment that variable.

What do y'all think? Is this a reasonable approach? Is there something I'm 
missing? A better way to proceed?

Thanks,
Jack