Add CleanStart source configuration

[cleanstart-community-admin]

This PR adds CleanStart as a git-based OSV source.

• Reuses APK ecosystem helper (similar to Alpaquita)
• Adds CleanStart source entry to source.yaml
• Removes purl as suggested by reviewer

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[another-rex]

Please separate out the source.yaml and source_test.yaml into separate PRs. (3 separate PRs)

This is because we need to merge and deploy _ecosystem.py first to support your records.
Then merge source_test.yaml to have it show up on our test instance.
Then once we both check that it is importing and rendering correctly, merge source.yaml to have it start ingesting in production.

[cleanstart-community-admin]

@another-rex
we have raised 3 separate PRs as requested:
#4646
#4647
#4648
Please review and let us know if any changes are required.

[another-rex]

Thanks! Closing this one then to look at the others.