firtool 1.131.0 generates references to non-existant layer files

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When generating a single verilog file, the layer files are included, even if they are not generated, leading to errors.

1. untar [no-include.tar.gz](https://github.com/user-attachments/files/22542462/no-include.tar.gz)
2. Run `firtool '--format=mlir' --disable-all-randomization -strip-debug-info '-disable-layers=Verification' '-disable-layers=Verification.Assert' '-disable-layers=Verification.Assume' '-disable-layers=Verification.Cover' counter_fir.fir -o counter.sv`

Now we see that references to non-existant files are generated:

```
$ grep include counter.sv
`include "layers-TestCounter-Verification.sv"
`include "layers-TestCounter-Verification.sv"
`include "layers-TestCounter-Verification.sv"
$ find . | grep layers
$
```

Extract no-include.tar.gz and run the firtool command from the issue to reproduce the generated counter.sv output. Trace the layer-include generation path for the disabled Verification layers and determine why references remain when no layer files are emitted. Done means counter.sv contains no references to those nonexistent layer files, with an appropriate regression check if the existing test structure is found.

Written by the indexing model from the issue text.