[FIRRTL] Missing Verifier that External Modules have Unique Names

Consider the following:

``` mlir
firrtl.circuit "Foo" {
firrtl.extmodule @Foo(
in a: !firrtl.uint<1>,
out a: !firrtl.uint<1>
)
}
```

This isn't tripping any verifier error:

``` console
# circt-opt Foo.reduced.0.mlir
module {
firrtl.circuit "Foo" {
firrtl.extmodule @Foo(in a: !firrtl.uint<1>, out a: !firrtl.uint<1>)
}
}
```

Apparently, this is only being checked in the parser:

``` console
# circt-translate -export-firrtl Foo.reduced.0.mlir | firtool -parse-only -format=fir
:5:12: error: redefinition of name 'a'
output a : UInt<1> @[Foo.reduced.0.mlir 4:9]
^
:4:11: note: previous definition here
input a : UInt<1> @[Foo.reduced.0.mlir 3:8]
^
```

Start with the FIRRTL verifier path exercised by circt-opt and compare it with the export path used by circt-translate. Reproduce the duplicate input/output name example; done means verifier diagnostics reject external modules with non-unique names before export.

Written by the indexing model from the issue text.