Test that the python_generate_pyi_srcs directive will add pyi_srcs to
generated targets and that it can be toggled on/off on a per-package basis.
The root of the test case asserts that the default generation mode (package) will compile multiple .pyi files into a single py_* target.
The per_file directory asserts that the file generation mode will attach
a single .pyi file to a given target.
Lastly, the per_file/turn_off directory asserts that we can turn off the
directive for subpackages. It continues with per-file generation mode.