Rename files to have matching prefixes

[Cjkjvfnby]

Describe your change:

Rename bfs to breadth_first_search so all files related to this algorithm are grouped together.

All files related to the algorithm:

breadth_first_search.py
breadth_first_search_2.py
breadth_first_search_shortest_path.py
breadth_first_search_shortest_path_2.py
breadth_first_search_zero_one_shortest_path.py

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.