`--multimodule` with duplicate `entry_point_name`s outputs one file

Running `cargo gpu --multimodule` on something like

```rust
#[spirv(fragment(entry_point_name = "main"))]
main_frag(){}

#[spirv(vertex(entry_point_name = "main"))]
main_vertex(){}
```

outputs a singular file (`main.1.spv`). I expect to have multiple file outputs.