Cleanups for fully typed build_target arguments

[dcbaker]

This is cleanups made possible by the fully typed build_target arguments.

The first patch moves some things that apply to jar into the shared-with-jar state. This was hidden by the allow_unknown=True.

Then there's some cleanups for the simd module since we can now be sure that no extra keyword arguments are passed

Finally, we can clean up the python module and get the fully type safe.