Remove mod.rs files (#1774)

### Description

Refactor the module system to be the same within a project

### Type of change

- [X] Refactor (no updates to logic)

### Rationale

We use two different ways to address the module system - without
`mod.rs` (only with directories/files) and with `mod.rs` in some places.
Unless there's a specific reason for that - one OR the other must be
used.
I refactored the code and removed `mod.rs` files.

### How Has This Been Tested?

I did `cargo build --release`

Author: curryrasul