[firtool] New blackbox/external-module behavior in firtool-1.137.0

Nobody has claimed this yet.

After updating from firtool-1.136.0 to firtool-1.137.0, I observed two changes in behavior to how external modules are handled, and I wasn't sure if they were intentional.

# Setup

Let's say I have this project structure:

```
./
├── bb-a/
│ └── Bar.sv
├── bb-b/
│ ├── Bar.sv
│ └── Baz.sv
└── Foo.fir
```

With these files:

```
$ cat bb-a/Bar.sv
// bb-a/Bar.sv
module Bar;
endmodule
$ cat bb-b/Bar.sv
// bb-b/Bar.sv
module Bar;
$ cat bb-b/Baz.sv
// bb-b/Baz.sv
module Baz;
endmodule
endmodule
$ cat Foo.fir
FIRRTL version 6.0.0
circuit Foo :%[[
{
"class":"firrtl.transforms.BlackBoxPathAnno",
"target":"~|Bar",
"path":"bb-a/Bar.sv"
},
{
"class":"firrtl.transforms.BlackBoxPathAnno",
"target":"~|Baz",
"path":"bb-b/Baz.sv"
},
{
"class":"firrtl.transforms.BlackBoxPathAnno",
"target":"~|Baz",
"path":"bb-b/Bar.sv"
}
]]
layer Verification, bind, "verification" :
layer Assert, bind, "verification/assert" :
layer Temporal, inline :
layer Assume, bind, "verification/assume" :
layer Temporal, inline :
layer Cover, bind, "verification/cover" :
layer Temporal, inline :

extmodule Bar :
defname = Bar

extmodule Baz :
defname = Baz

public module Foo :
input clock : Clock
input reset : UInt<1>

inst bar of Bar
inst baz of Baz
```

So, I've got a duplicate definition of the SystemVerilog `Bar` module in both sub-directories, and I've included `bb-a/Bar.sv` as the definition of the `Bar` FIRRTL module, and I've added `bb-b/Bar.sv` as a file for the `Baz` FIRRTL module, even those it's not going to be useful in the circuit definition.

# Strict handling of duplicate module definitions

When I run this with firtool-1.136.0, it compiles, and I get this result:

```
$ ./firtool-1.136.0 -default-layer-specialization=enable Foo.fir
// Generated by CIRCT firtool-1.136.0
module Foo(
input clock,
reset
);

Bar bar ();
Baz baz ();
endmodule

// ----- 8< ----- FILE "./Bar.sv" ----- 8< -----

// Generated by CIRCT firtool-1.136.0
// bb-a/Bar.sv
module Bar;
endmodule
// ----- 8< ----- FILE "./Baz.sv" ----- 8< -----

// Generated by CIRCT firtool-1.136.0
// bb-b/Baz.sv
module Baz;
endmodule
```

But with firtool-1.137.0, the build fails:

```
$ ./firtool-1.137.0 -default-layer-specialization=enable Foo.fir
Foo.fir:30:3: error: redefinition of symbol named 'Bar.sv'
extmodule Baz :
^
Foo.fir:30:3: note: see current operation:
"emit.file"() <{file_name = "./Bar.sv", sym_name = "Bar.sv"}> ({
"emit.verbatim"() <{text = "// bb-a/Bar.sv\0Amodule Bar;\0Aendmodule\0A"}> : () -> ()
}) {output_file = #hw.output_file<"./Bar.sv">} : () -> ()
Foo.fir:27:3: note: see existing symbol definition here
extmodule Bar :
^
```

Now, this isn't necessarily a bad change, but is this an expected change in behavior? Or a weird edge-case I'm hitting? Is this a change that we should communicate to users?

# New unprocessed-annotation warning

If we run this same example with the `-warn-on-unprocessed-annotations` flag, there's no warning in firtool-1.136.0:

```
$ ./firtool-1.136.0 -default-layer-specialization=enable -warn-on-unprocessed-annotations Foo.fir
... same output as above
```

But with firtool-1.137.0:

```
$ ./firtool-1.137.0 -default-layer-specialization=enable -warn-on-unprocessed-annotations Foo.fir
Foo.fir:27:3: warning: unprocessed annotation:'circt.VerbatimBlackBoxAnno' still remaining after LowerToHW
extmodule Bar :
^
... same error output as above
```

And if I update the project to remove `bb-b/Bar.sv`, and remove that annotation from `Foo.fir`, I still get the warning with a successful result:

```
$ ./firtool-1.137.0 -default-layer-specialization=enable -warn-on-unprocessed-annotations Foo.fir
Foo.fir:22:3: warning: unprocessed annotation:'circt.VerbatimBlackBoxAnno' still remaining after LowerToHW
extmodule Bar :
^
// Generated by CIRCT firtool-1.137.0
module Foo(
input clock,
reset
);

Bar bar ();
Baz baz ();
endmodule

// ----- 8< ----- FILE "./Bar.sv" ----- 8< -----

// Generated by CIRCT firtool-1.137.0
// bb-a/Bar.sv
module Bar;
endmodule

// ----- 8< ----- FILE "./Baz.sv" ----- 8< -----

// Generated by CIRCT firtool-1.137.0
// bb-b/Baz.sv
module Baz;
endmodule
```

Is that warning expected?

# Reproduction

Here's a folder containing those files, if you want to reproduce the same results:

[blackbox-errors.tar.gz](https://github.com/user-attachments/files/23941654/blackbox-errors.tar.gz)

Start with the blackbox-errors.tar.gz reproduction and run Foo.fir through firtool-1.136.0 and firtool-1.137.0 using -default-layer-specialization=enable and -warn-on-unprocessed-annotations. Compare duplicate external-module handling and the circt.VerbatimBlackBoxAnno warning. Done means determining whether both behavior changes are intentional and identifying the expected user-facing outcome.

Written by the indexing model from the issue text.