transpile: handle module name collisions

[randomPoison]

In testing against libgit2, we ran into a case where module names could collide, leading to a generated Rust file not being included in the build. The situation is that we have both a hash.c file and some files in a hash directory, e.g. hash/sha1.c. C2rust then wants to generate both

mod src {
    mod hash;
}

and

mod src {
    mod hash {
        mod sha1;
    }
}

In this case, the latter is what gets generated, effectively dropping src/hash.rs from the Rust build.

This PR has c2rust detect these collisions and rename the colliding module, so we'd generate something like

mod src {
    #[path = "hash.rs"]
    mod hash_;
    mod hash {
        mod sha1;
    }
}