restructure modules to be canononical and for readability

[liamsi]

Currently, we use the file and folder structure from startport. What we should be doing is rather using the recommended folder structure here: https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md#recommended-folder-structure

Where it helps readability we should deviate from that. I think Osmosis did a great job structuring their modules: https://github.com/osmosis-labs/osmosis/tree/main/x

A good opportunity to tackle this, would be when we upgrade to tendermint 0.35 and 0.46 of the SDK (related to celestiaorg/celestia-core#630 (comment)).

[evan-forbes]

leaving a comment to remind us to also fix #472 (comment) when we address this issue

[rootulp]

Closing this because we have stand-alone modules in the x/ directory