Simplify simulation packages

[julienrbrt]

Summary

We should converge to 2 simulation packages, one for app simulation and one for module simulations.

Problem Definition

Currently, we have 3 simulation packages.

• github.com/cosmos/cosmos-sdk/x/simulation
• github.com/cosmos/cosmos-sdk/simsx
• github.com/cosmos/cosmos-sdk/types/simulation
• github.com/cosmos/cosmos-sdk/testutil/sims

Work Breakdown

We can do this simplification on main already.
Once the simplification is completed, close the no more relevant issues: https://github.com/cosmos/cosmos-sdk/issues?q=is%3Aissue%20state%3Aopen%20simulation%20label%3AC%3ASimulations