Refactor (moved utils fns to traversal.py)

[akshatmardia]

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Working on refactoring the code according to the TODO comments found:

TODO found in /youtube_dl/traversal.py which says TODO: move these utils.fns here and move import to utils.

I have moved the functions and added the imports in /youtube_dl/utils.py but flake8 does not like that they are not all being used in utils.py but being imported in other files via utils.py. Note that the tests pass for both utils.py and traversal.py.

Should I add traversal.py imports in those separate files or just ignore the flake8 error and let them be imported from utils.py?

This PR ignores the flake8 error with #noqa

[akshatmardia]

@dirkf can I get a review please?