Subpackages in modules

[rigelrozanski]

This issue is specifically referencing how complex staking is, and that we should split in up.

General Proposed Design Approach:
In the same vein as #1123 - /x modules with substantial amounts of logic should probably be split into multiple packages - but then aliases created in x/your-module/types.go so that as an external user of the module, I don't need to understand the inner structures of effectively reference it's types

CC: @cwgoes

[cwgoes]

We shouldn't overuse this, but I agree that it makes sense right now for x/staking, and could make sense in the future for other modules of similar complexity.

[sunnya97]

@rigelrozanski Can we close this issue?

[rigelrozanski]

Not prelaunch, but I think we should keep open - I think both slashing and gov should be reorganized to use subdir. As gov expands with more features this will become more useful as well.

[jackzampolin]

Going to close this as addressed by the module refactor.

[rigelrozanski]

only once #4296 has been merged should this issue be closed (which is will be automatically by the PR right)