Add/update cocoapods and pypi download_url (and other) support to fetchcode/package.py

In connection with a [purl2url issue](https://github.com/package-url/packageurl-python/issues/143) in `packageurl-python`, we cannot add download URL support for `cocoapods` or `pypi` in `purl2url.py` because the process involves network calls, and consequently will need to add/update that support in `fetchcode/package.py`.