Add submodule to sys.path when loading child idl file.

[cocolato]

Fixes: https://github.com/Thriftpy/thriftpy2/issues/215

I'm not sure if it's suitable to add the module directly to sys.path, maybe is should implement like this:

for module in thrift.__thrift_meta__["includes"]:
    if module not in sys.modules:
        sys.modules[module.__name__+"_thrift"] = module

then use the import hook to make sure pickle will import the suitable module.

[cocolato]

Now has been updated to a recursive implementation.

[aisk]

I have some concern about whether this code can handle circle including IDLs. Can you add a test case to ensure this?

[cocolato]

Ok, i'll test this part later.

[cocolato]

I have some concern about whether this code can handle circle including IDLs. Can you add a test case to ensure this?

There are already have tests for dead inclusions. https://github.com/Thriftpy/thriftpy2/blob/91b28ea3de4bc3d734364d9e902b8a2ddee5fc80/tests/test_parser.py#L201-L204