_stream.py: Ensure source-bundle's source directory exists

Currently, `source-bundle` command is entirely broken as it tries to stage the
sources in a directory that doesn't exist. Fix it by ensuring that we create
the necessary directories before calling any methods that try to use those
directories.

This fix comes with a regression test to ensure that the basic use-case
of `source-bundle` continues to work in future.

Fixes #651.