refactor importer_yielder.py

[Hritik14]

Do we really need this file ? Could the importer classes help enclosing these information ?
@pombredanne can you shed a little more light here ?

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[Hritik14]

Dumping importer_yielder

• Simply store data source as concrete lists
• Don't store configuration or last run, maybe we can have better Log Importer Runs later. Ticket: Log Importer Runs #526

Consider: https://github.com/nexB/scancode-toolkit/blob/develop/src/packagedcode/__init__.py#L41

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