add support for src/main.rs layout

[davidB]

Add support for a layout like:

src/
+-- main.rs
+-- mymod.rs

The main.rs support only private "mod" listed in the main.rs but they are converted into "pub mod" to limit the change in the existing code.

Today I used the following project template ton codingame, https://gitlab.com/davidB31/cg-rustkit/tree/master/starter with the python bundler.