Fix/internal event datastore dep

[masl2]

Description

Introducing the letter to letter event mapper into the event package created a dependency on the internal/datastore to support the typing.

However, external importers (digital letters) do not have access to this internal dependency. Therefore the mapper has been moved out of the event package.

To avoid circular package dependencies this has been placed into a new internal package

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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