Summary

This is a fairly simple mod that I originally made for myself, though given the enormous collection of available mods for Skyrim, I don't doubt that there is already something that offers at least fairly similar functionality.
However, if you want a lightly scripted solution for this that is light on compatibility issues, then you might consider if this works for you.

Description

The idea was to find a way to access the Alchemy ingredients that a resourceful Dragonborn would be able to harvest from defeated foes, and particularly some of the harder-to-find ingredients.
Fallout 4 has the excellent Manufacturing Extended mod, which provides an Auto-Butcher that breaks down entire bodies, particularly to provide easier access to the 'Bone' component.
Unfortunately, Skyrim is a bit limited both in terms of available bone items that can be collected, and also in terms of the 'death item' available for most generic NPCs.
As such, it would have been infeasible to attempt to add death items for different actors, especially because every mod introducing new NPCs would require compatibility patches.

Instead, this mod uses a scripted system to allow the player to harvest the appropriate ingredients.
A "[Tool] Harvesting Kit" 'armor' piece is available to craft. Equipping this will add a hidden perk to the wearer that adds an activate option (this is a perk entry point in the game) for harvesting corpses.
Additionally, if the wearer has the "Green Thumb" Alchemy perk, then the looted item pool is slightly improved.
The kit doesn't take any armor slots, and so should always be available to wear.

The item returns are as follows:
Humans:

• 2 Bone Meal
  

• 1 Skull
  

• 1 Human Heart
  

• 1 Human Flesh
  

Humans (with Green Thumb):

• 3 Bone Meal
  

• 1 Skull
  

• 1 Human Heart
  

• 2 Human Flesh
  

Human vampires also yield 1 Vampire Dust, in addition to the contents of either above list.
(Human) Forsworn Briarhearts have the same lists as humans, but do not yield Human Hearts.

Other Humanoids (i.e. Elf races):

• 2 Bone Meal
  

Other Humanoids (with Green Thumb):

• 3 Bone Meal
  

Vampires of these races yield, like humans, 1 Vampire Dust in addition to the items of their appropriate list.

Compatibility

As mentioned, a major focus for this mod was compatibility. This mod requires neither any DLC nor the script extender, but should still be compatible with most added NPCs.
One important point, though, is that this only works with the races available in the vanilla game. As such, any custom races will not allow for harvesting items.
The structure of the mod is such that custom races could be added if necessary, but the mod as is should accommodate the wide majority of possible use scenarios.

A minor incompatibility is possible with mods that significantly alter the Alchemy perk tree, but provided that the base "Green Thumb" perk record still exists, there shouldn't be any huge issues.
Incidentally, I tested this with CACO and it works just fine.

No vanilla records are overridden, and no scripts are attached to actors. This mod simply checks for having already harvested corpses by the presence of a special, non-playable (invisible) token item.

Disclaimers

This mod is lightly scripted, and has no constantly running scripts, heavy OnUpdate() events, or anything of the sort.
That said, there is still an important script attached to the harvesting kit that adds the necessary perk to the wearer when equipped, and removes it when unequipped.
Try not to fiddle with this too much, especially e.g. by attempting to rapidly equip and unequip multiple different copies of the kit at once.
The recipe for the kit is conditioned on the creator having no kit already in their inventory, to discourage having multiple copies of it at once.

Although I consider this unlikely to be necessary, if you really get stuck with the perk either added or removed, follow these steps:

• Remove every kit from your inventory
  

• Open the console and run 'help Rdr_HarvestingPerk 0'
  

• Find the corresponding FormID for the perk, and run 'player.RemovePerk xx000D63' (where xx represents the load order of this mod as found in the previous step)
  

This should clear out the perk from the player, and allow the kit to properly add it the next time it is equipped.

I wrote the perk script in such a way as to be efficient with marking a target corpse as already harvested.
However, there is a chance that, if the user tries to harvest a corpse multiple times very quickly, a second activation could become available before previous one has deposited it's token.
The solution is just to not try exploiting the system like that. Adding the items to the player both displays a notification in the corner of the screen, and plays a sound effect, which indicates that the procedure is complete.

The usual warnings for mods apply - I am not responsible for anything bad that may happen to your saves/game/computer/etc., use at your own risk, ...

Please do not upload this elsewhere.