[FIRRTL] Handle/Disallow Layer-under-layer that creates bind-under-bind

Nobody has claimed this yet.

The current lowering of the following produces illegal SystemVerilog that does bind-under-bind:

```
FIRRTL version 4.0.0
circuit Foo: %[[
{"class":"firrtl.transforms.DontTouchAnnotation", "target":"~Foo|Bar>b"},
{"class":"firrtl.transforms.DontTouchAnnotation", "target":"~Foo|Foo>a"}
]]
layer A, bind:
layer B, bind:

module Bar:

layerblock B:
wire b: UInt<1>
invalidate b

module Foo:

layerblock A:
wire a: UInt<1>
invalidate a

inst bar of Bar
```

This produces:

```verilog
// Generated by CIRCT firtool-1.64.0-25-g3dbdd0bc9
module Bar_B();
wire b = 1'h0;
endmodule

module Bar();
endmodule

module Foo_A();
wire a = 1'h0;
Bar bar ();
endmodule

module Foo();
endmodule

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

// Generated by CIRCT firtool-1.64.0-25-g3dbdd0bc9
`ifndef groups_Foo_B
`define groups_Foo_B
bind Bar Bar_B bar_B ();
`endif // groups_Foo_B

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

// Generated by CIRCT firtool-1.64.0-25-g3dbdd0bc9
`ifndef groups_Foo_A
`define groups_Foo_A
bind Foo Foo_A foo_A ();
`endif // groups_Foo_A
```

There are two issues:

1. This should be caught by a verifier after the bind-under-bind is created: https://github.com/llvm/circt/issues/5530
2. The FIRRTL pipeline should disallow this with a verifier in possibly one of two ways:
a. Layerblocks which instantiate modules which transitively have layers should be made illegal.
b. `LowerLayers` needs to work harder to split modules created from layerblocks to avoid this situation entirely.

For 2.b this is hinting that module `Bar` needs to be factored into two units: One which contains only layer A code and one which contains both A and B. Only the one with A code should be bound into `Foo`.

h/t @youngar for identifying this problem

Start with the FIRRTL LowerLayers pipeline and the verifier related to bind-under-bind, using the Foo/Bar reproducer in the issue to reproduce the illegal SystemVerilog. Compare the two proposed directions: rejecting layerblocks that transitively instantiate layers, or factoring the lowered modules so only the valid layer code is bound. Done means the reproducer no longer permits illegal bind-under-bind output.

Written by the indexing model from the issue text.