Support for building stdlib packages

[seddonym]

Currently, grimp fails to build a graph of a stdlib package:

>>> import grimp
>>> g = grimp.build_graph('sys')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../lib/grimp/src/grimp/application/usecases.py", line 26, in build_graph
    file_system=file_system,
  File ".../lib/grimp/src/grimp/adaptors/packagefinder.py", line 23, in determine_package_directory
    assert package_filename.origin  # For type checker.
AssertionError