Migrate qiskit_nature.transformers

[mrossinek]

What should we add?

As part of #701, all existing modules of Qiskit Nature will be migrated.
This issue outlines the requirements for the existing transformers module.
This issue is blocked by #706.

⚠️ This migration needs to copy the code in order to abide the deprecation process!

Below are the current module contents:

qiskit_nature/
    transformers/
        second_quantization/
            BaseTransformer
            electronic/
                ActiveSpaceTransformer
                FreezeCoreTransformer

Here are the new locations for all of these classes.

qiskit_nature/
    second_quantization/
        problems/
            BaseTransformer
            electronic/
                transformers/
                    ActiveSpaceTransformer
                    FreezeCoreTransformer

Unittests should be migrated accordingly.