modules/keymap: improve lua deprecation

[MattSturgeon]

• Replace nullable lua option with a no-default option.
• Made it so the deprecated option is only declared when lua = true is passed.
• Replace normalizeMappings with a removeDeprecatedMapAttrs helper.
• Added warnings for all options that historically had lua support.

This supersedes and closes #1795

Removing the attr from the final value

I'd like to do something like this directly in mkMapOptionSubmodule, however it doesn't seem to do what you'd expect...

type // {
  merge = loc: defs: builtins.removeAttrs (type.merge loc defs) [ "lua" ];
};

Therefore I've instead added apply functions to each keymap option that removes the lua attr.

See this message on matrix.

Showing the warning for all uses

Previously, we only checked the keymaps option in our deprecation warning.

We could instead have a lib.warn in the action's apply function when we call mkRaw, however this wouldn't have access to loc or defs to print a proper warning.

If overriding the merge function worked correctly, we could do it there, however I think this is fine for this PR, we can always improve things further in future work.

For now, modules/keymaps.nix is responsible for showing warnings for all options that previously had a lua sub-option.

[khaneliman]

Feels weird that the merge would be completely ignored, resulting in needing to apply in every consumer of a function. But, in lieu of a better alternative it makes sense to me, for now.

With the conversation around #1935 will this be another place we have a migration away from recommending __raw

[MattSturgeon]

Feels weird that the merge would be completely ignored

I don't fully understand it either, we had the same issue implementing transitionType, which you'd expect could just override coercedTo's merge function.

My best guess is something in mkOptionType is using the merge passed to it, rather than the merge in the final attrs... Maybe this is something that could be resolved in nixpkgs?