Fix nested workunits

[WeiTaShao]

Implicit helper functions

Standalone PyKokkos workunits can call compatible module-level Python functions without requiring the @pk.function decorator. PyKokkos discovers helper functions reachable from the workunit, recursively includes their same-module dependencies in translation, and excludes unrelated host-side functions.

import pykokkos as pk


def my_printer(i: int) -> None:
    pk.printf("Index: %d\n", i)


@pk.workunit
def hello(i: int) -> None:
    my_printer(i)


pk.parallel_for("hello_loop", 10, hello)

[IvanGrigorik]

self.add_called_functions is highly questionable.
I'm not sure if this is the easiest way to check which function is called.

Instead of gathering all possible functions (which is insanely heavy) we can check which function is called based on the current context directly from the AST translator.

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And yes, don't remote @pk.function decorator nor ignore it. It will be a pain if we will try to visit each function without looking at its source of it.

[IvanGrigorik]

So...
Now you will recompile if ANYTHING in AST changed?
That's questionable.

[IvanGrigorik]

Please add a test of defining a function in one module (file) and using it from another module workunit.