[RFC] Plugin/Firmware Discoverability

[makefu]

Right now nixpkgs has a number modules which provide a plugin system for adding extra functionality to the service. Typical examples are:

• services.printing.drivers - print drivers
• hardware.sane.extraBackends - scanner drivers
• services.pcscd.plugins - drivers for the smart card reader
• boot.extraModulePackages - modules which should be loaded on startup

Issue description

Even though we have the plugin system, it is opaque which packages can actually be used as a plugin for the respective module. The modules are often scattered around in different folders (most notably with services.pcscd.plugins ).
Most of the time i simply grep the nixpkgs sources for keywords which may relate to the plugin i am looking for.

Steps to reproduce

1. Try to find all plugins for pcscd (there are at least 4 on the stable channel)
2. Try to find all backends for sane

Technical details

I do not know how what the best way would be to solve this issue, not all packages which can be plugins are only plugins but provide binaries or libraries as well. Maybe we could introduce a meta attribute which hints towards the package(s) or module(s) which can ingest them as plugins.
In the options search this information could be used to backtrace which extra packages are available.

[symphorien]

Most if not all sane backends are documented in the wiki https://nixos.wiki/wiki/Scanners

[stale]

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