Add report-external command

[emdoyle]

cc: @devstein

This PR introduces report-external, which detects usages of third-party packages in a given file path. It can also be used with --raw to generate machine-readable output suitable for piping into a requirements.in during a build process.

Example:

> tach report-external --raw python/tach
pyyaml
pydantic
stdlib-list
importlib-metadata
gitpython
prompt-toolkit
networkx
rich

[devstein]

@emdoyle 🙇‍♂️