`entry_point_name` should be decoupled from the output file name

When using something like this

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

I expect output to be main_frag.spv with an OpEntryPoint of "main". Instead, I am seeing cargo gpu using entry_point_name for the output file name. This means I can't decouple them.

I need to decouple as I have a multishader, so I need to output unique file names to know which shader is which. I am running in a harness I don't control that expects "main" for all shader entry points.

Start by tracing cargo gpu's output filename handling for the entry_point_name attribute. Add a regression test covering a function such as main_frag with entry_point_name = "main"; done means the output file uses the function name while the SPIR-V OpEntryPoint retains "main".