workerutil: Move store into own package

Created by: efritz

This solves a common issue with mock generation where tests require a mock of something defined in its own package. Having a separate mocks package would add a cyclic dependency.

This PR moves the workerutil store into its own package so we can generate a mock for it (which is needed by the precise code intel indexer). Callers do not need to be aware of this change as we import all the relevant exports from the new package into the workerutil base package.