Bumper-lib

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* Return bumps made for BumperDriver.bump()

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* Only check existing requirement for one that will be attempted to be bumped

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* Only bump once per package and check requirements after all bumps are done to ensure all requirements are met

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* Fix bump for package name / no version
* Fix bumping to specific version

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* Add should_add to check if a product name should be added to the bumper target

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* Return empty changes list instead of None