Allow model piping within a multi-target

[billdenney]

Allow automatic detection of being in the same target to allow the model to be found, chosen, and passed into a pipe.

[mattfidler]

This is what I was going to test for in the review. Let me know when this is done so I can test it and look to see if it is working well.

[billdenney]

@mattfidler, I just merged the changes for this. It should now work with model piping within a multiple-model fitting call. It should also detect circular (or hanging) references to models that do not exist or cannot be created.

I added it to the vignette: https://nlmixr2.github.io/nlmixr2targets/articles/estimating.html#model-piping-for-multiple-models-estimated-with-one-dataset