Improve `hash_callable` to include function dependencies in hash calculation

[ilongin]

Current implementation of hash_callable() function calculates hash of a function by looking into function's source code. But if original function calls some helper functions defined in the same module, and those helper functions change this won't affect the hash calculation of original function - hash will stay the same.

We should change this to include helper function calculation as well. Built-ins, imported functions should be ignored.