Enhance the Agda builder

[michaelpj]

For my sins, I'm trying to integrate some Agda code into the testing part of a production system. I'd like to use the Agda builder from Nixpkgs, but it could do with a few enhancements. I thought I'd open an issue discussing those so I can see whether other people agree before I try to actually implement anything.

Things I would like:

• Support for library files.
  • The current builder passes all dependencies using -i, we could probably use AGDA_DIR or --library-file now for many libraries.
  • Would need to think of how to handle fallback for libraries that don't use library files yet.
• agdaWithPackages
  • Obviously we'd want to preserve the existing ability for emacs to pick things up, but I think might be easier with AGDA_DIR now.
• Easier configuration of build flags.
  • It's a bit annoying to try and override buildFlags at the moment.
• Some simple enable/disable options for generating html/latex literate Agda output, and for producing compiled outputs via one of the backends.

Mentioning some people who seem to be involved: @Fuuzetsu @siddharthist

[Fuuzetsu]

I haven't used Agda for few years so I'm not really up to date on the current build system in nix but the suggestions sound fine I think.

[langston-barrett]

See also: https://github.com/srdqty/nix-agda-with-packages

[turion]

@langston-barrett can your project be merged into nixpkgs? Also, there is a PR here: #39318