What is the name of the rigid plastic cord in this hem?
Welt cord and piping cord are the correct terms.

They come in different varieties, with the center filling being made of different materials, such as foam or rope or cord. 

You can also get them hollow, and then you buy a cord of your choice to thread through it, such as the rigid plastic cord that you'd like.

Instead of buying pre-made piping cord, you can also make your own. Again, you end up filling the "pipe" with a cord of your own choosing. 

If you choose a rigid plastic cord, you'll lose some flexibility. Which, in a pack/bag, I imagine is the point, as it will retain its shape better when empty.