Which triggers first: steel trap or health pack?
It is dependent on which area of effect you enter first, if you enter both simultaneously (i.e. with Sombra's translocator) the health pick-up will trigger first.
A 5hp Sombra translocates onto a Health pickup:
Notice how the Sombra heals to full, and then has damage from Junkrat's trap applied.
Larger gfycat of above gif.