README: clarify extraEmacsPackages usage

[acid-bong]

Inspired by mu4e's recipe

Now here's a lil issue i'd love to resolve:

• if you nix build|run such a closure, extraPackages won't be available
• only when this closure is installed (with nix profile install or adding it to environment.systemPackages/home.packages), extraPackages can be propagated.*

Is this behaviour fine as is or should I make extraPackages always available?

*Just tried with nixos-rebuild test, somehow this doesn't work 🤔

[talyz]

Another issue is that this only works when defaultInitFile is used, which it isn't by default.

[jian-lin]

I am strongly against using propagatedUserEnvPkgs here. The reasons are as follows:

• propagatedUserEnvPkgs has issues. See links in [documentation] There is no documentation for propagatedUserEnvPkgs NixOS/nixpkgs#347093.
• Here, we use emacsWithPackages which will add bin dirs of packages in extraEmacsPackages to exec-path. To find executables, Emacs lisp packages should respect exec-path by using functions like executable-find. If one Emacs lisp package do not respect exec-path, that package itself should be fixed.
• I use propagatedUserEnvPkgs in the mu4e package only to support unrecommended use cases where emacsWithPackages is not used. For example, users install Emacs lisp packages by adding them to environment.systemPackages.

[acid-bong]

Ight, I'm limiting this edit only to README, where i clarify that one can just use extraEmacsPackages or .withPackages. Or, better, just rename the former

[jian-lin]

LGTM, thanks.