Add checks for nested tasks

[zhanyuanucb]

Describe
Add check for nested tasks.

To Reproduce
A nested tasks in a vanilla function:

from pirlib.pipeline import pipeline
from pirlib.task import task
from pirlib.iotypes import DirectoryPath, FilePath

test_file_path = FilePath("test/file.txt")

def func(inp: FilePath) -> FilePath:
    @task
    def my_task(inp: FilePath) -> FilePath:
        return inp
    return my_task(test_file_path)

func(test_file_path)

See the error

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in func
  File "/pirlib/pirlib/task.py", line 113, in __call__
    return self.instance(self.name)(*args, **kwargs)
  File "/pirlib/pirlib/package.py", line 258, in wrapper
    return func(instance, *args, **kwargs)
  File "/pirlib/pirlib/task.py", line 67, in __call__
    outputs = backend.execute(package, self.name, self.config, inputs=inputs)
  File "/pirlib/pirlib/backends/inproc.py", line 72, in execute
    node_outputs[node.id] = self._execute_node(node, node_inputs)
  File "/pirlib/pirlib/backends/inproc.py", line 88, in _execute_node
    handler = getattr(importlib.import_module(module_name), handler_name)
AttributeError: module '__main__' has no attribute 'my_task'

A nested task in a pipeline:

from pirlib.pipeline import pipeline
from pirlib.task import task
from pirlib.iotypes import DirectoryPath, FilePath

test_file_path = FilePath("test/file.txt")

@pipeline
def my_pipeline(inp: FilePath) -> FilePath:
    @task
    def my_task(inp: FilePath) -> FilePath:
        return inp
    return my_task(test_file_path)

my_pipeline(test_file_path)

See the error

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/pirlib/pirlib/pipeline.py", line 88, in __call__
    return self.instance(self.name)(*args, **kwargs)
  File "/pirlib/pirlib/package.py", line 177, in wrapper
    return method(instance, *args, **kwargs)
  File "/pirlib/pirlib/pipeline.py", line 37, in __call__
    package = package_pipeline(self.defn)
  File "/pirlib/pirlib/package.py", line 147, in package_pipeline
    _pipeline_to_graph(definition.func, definition.name, definition.config)
  File "/pirlib/pirlib/package.py", line 164, in _pipeline_to_graph
    return_value = pipeline_func(*args, **kwargs)
  File "/miniconda3/envs/pirlib/lib/python3.8/site-packages/typeguard/__init__.py", line 1033, in wrapper
    retval = func(*args, **kwargs)
  File "<stdin>", line 6, in my_pipeline
  File "/pirlib/pirlib/task.py", line 113, in __call__
    return self.instance(self.name)(*args, **kwargs)
  File "/pirlib/pirlib/package.py", line 268, in wrapper
    inputs=_inspect_inputs(instance.func, args, kwargs),
  File "/pirlib/pirlib/package.py", line 220, in _inspect_inputs
    recurse_hint(add_input, param.name, param.annotation, value)
  File "/pirlib/pirlib/package.py", line 70, in recurse_hint
    return func(prefix, hint, *values)
  File "/pirlib/pirlib/package.py", line 210, in add_input
    source=value.source
AttributeError: 'FilePath' object has no attribute 'source'

Expected behavior
Either support this behavior to throw exceptions if find them

Environment (please complete the following information):

• OS: MacOS
• Python 3.8