Refactor downloader.py to make more functions unit testable

[kollivier]

Description

Currently, there are a lot of programming practices, like inner functions, in downloader.py that prevent us from unit testing features such as URL detection and re-writing. This leads to whack-a-mole bug fixing since we don't have the resources to test all existing sources with any changes we make.

[Tharun-10Dragneel]

hi is the issue is still open?i would like to work on this issue!

[learning-equality-bot]

Hi! 👋

Thanks so much for your interest! This issue is not open for contribution. Visit Contributing guidelines to learn about the contributing process and how to find suitable issues. If there are no unassigned 'help wanted' issues available, please wait until new ones are added.

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[jaltekruse]

@Tharun-10Dragneel I am planning to work on #655 Which is a generalization of this type of problem in the ricecooker code, and is now marked as superseding this issue. If you would like to contribute to that effort or follow along we could maybe connect on the project slack instance. I didn't see anyone named Tharun on there, if you haven't asked to join you can check out this page for getting started info on contributing https://learningequality.org/contributing-to-our-open-code-base/

[learning-equality-bot]

Hi! 👋

Thanks so much for your interest! This issue is not open for contribution. Visit Contributing guidelines to learn about the contributing process and how to find suitable issues. If there are no unassigned 'help wanted' issues available, please wait until new ones are added.

We really appreciate your willingness to help. 😊

Are you preparing for Google Summer of Code? See our GSoC guidelines.