simplify get/find_kernel_specs.

[Carreau]

listing all kernelspec ($ jupyter kernelspec list) make N+1 call to find_kernel_specs (one for each found kernel)

This could be highly simplified with a "find_all_specs".

Not important, but simple for beginner, so marking as sprint friendly.

[minrk]

👍 for a sprint. But you make it really hard for me to not do these things right away.

[Carreau]

If you want a motivation: you are paid not to do them, and to guide new comers.
I make efforts not to fix all the things too.

I can also assign someone else:

@parleur I know you are moving soon, but take a break on packing and contribute to Jupyter.

[captainsafia]

Lemme give this one a go.

[jankatins]

The notebook does something similar:
https://github.com/jupyter/notebook/blob/master/notebook/services/kernelspecs/handlers.py#L56