Initialize set with source in DFS (#1872)

* Update dfs.py

* Add type hints, rearrange doc-strings and comments

* fixup! Format Python code with psf/black push

* dfs -> depth_first_search

Co-Authored-By: Christian Clauss <cclauss@me.com>

* dfs -> depth_first_search

* Add doctest for DFS

* fixup! Format Python code with psf/black push

* Rename dfs.py to depth_first_search_dictionary.py

* updating DIRECTORY.md

* Rename depth_first_search_dictionary.py to depth_first_search_dfs.py

* updating DIRECTORY.md

* Rename depth_first_search.py to depth_first_search_2.py

* updating DIRECTORY.md

* Rename depth_first_search_dfs.py to depth_first_search.py

* updating DIRECTORY.md

Co-authored-by: John Law <johnlaw.po@gmail.com>
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>