Ensure plugins are dynamically loadable

[edwtjo]

This blog post describes how/why I ended up writing this PR.

[mantkiew]

I hit this issue trying to run gitit binaries on CachyOS with plugins. Can this be safely merged without causing any unexpected problems?

On CachyOS, you can do

pacman -S gitit

and just run prebuilt binaries. However, when trying to run with a plugin, you get this

Loading plugin '<my plugin here>'...
gitit: <interactive>:1:1: error:
    Could not find module `Network.Gitit.Interface'
    There are files missing in the `gitit-0.16' package,
    try running 'ghc-pkg check'.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.

That is because all Haskell packages on Arch/CachyOS are installed as dynamic only https://wiki.archlinux.org/title/Haskell
@jgm would it be possible to merge this PR?

[jgm]

I really have no idea. Gitit is only semi-maintained at this point, and it has been over a decade since I've looked at plugins. I would suggest trying to integrate the change and compiling from source, and let us know if it solves your problem.