Consider Declaring the Interface Factory locally

[EmilGeorgiev]

The interface mev.Factory is currently used in the package abci/checktx. I believe it would be better for the abci/checktx package to declare its own interface because:

1. The abci/checktx package doesn't use all the methods of the mev.Factory interface. It would be more efficient to declare an interface that suits its specific needs.

2. By declaring its own interface, the abci/checktx package can avoid importing the dependency on mev.Factory.

3. This approach follows Go best practices as outlined here Go Style Guide - Interfaces.