`entry_point_name` should be decoupled from the output file name

[LegNeato]

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.

[schell]

We should write a test for this one.