The `bevy_ecs::system::system` module is linting `clippy::module_inception`

While working on https://github.com/bevyengine/bevy/pull/17335, I came across `bevy_ecs::system::system`, which was linting [`clippy::module_inception`](https://rust-lang.github.io/rust-clippy/master/index.html#module_inception).

I believe one module or the other should be renamed, as the current naming is confusing.

Locate the bevy_ecs::system::system module and inspect the surrounding module structure and naming. Check the clippy::module_inception lint there, then determine which module name can be changed without obscuring the API. Done means the confusing module-inception lint is resolved and the resulting names are consistent.

Written by the indexing model from the issue text.