Deprecating having both `src/lib.rs` and `src/main.rs` in the same package

[epage]

I would be hesitant to shuffle around what we do for modules without first understanding if we'd want to further explore the idea at The module scheme (`./module.rs` file + `./module` folder instead of just `./module/mod.rs`) introduced by the 2018 edition maybe a little bit (more) confusing - #6 by josh