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

[LegNeato]

Running cargo gpu --multimodule on something like

#[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.

[schell]

This appears to be a bug in spirv-builder, as only one module is making it out of the call to SpirvBuilder::build.