What is the problem that needs to be solved (required)? There is no indication that the selected mod requires any other mod from the list.

What is the idea in abstract terms (optional)? Highlight the secondary/primary mods in some way.

Should point out the required primary/secondary mods in the mod info window as well.

Should the info be pointed out in the mod info window or in the modlist? If in the modlist, where and how?

Add (or replace the current warning icon with) a "stop" icon in the modlist if the required mods are not found?

Enumerate the required mods in the mod info window with either an "ok" or "broken" signs next to each entry according to their presence?

I'm looking for suggestions.

Those sound like good ideas.

I believe the original point of this request was that the primary and secondary mods are highlighted in the modlist itself.

Modlist highlights mods when a mod is selected and when a mod is activated. (implemented in 0.8.0).