[FIRRTL] extmodule as main module

Input:

```firrtl
circuit Ext :
extmodule Ext :
input x : UInt<1>
```

Produces:

```
// Generated by CIRCT 1.31.0g20230217_f4a1235
// external module Ext
```

We don't need the port, but SFC apparently does, which seems to also accept this (producing an empty file) as of 1.5.3, FWIW.

This isn't unreasonable behavior, but probably should be rejected? Kinda surprised (in a good way) nothing breaks along the way with this! :laughing:

Looks like `defname` can be used to rename the main module too, FWIW. :thinking:

Reproduce the FIRRTL input with an extmodule as the main module and inspect why CIRCT produces only the generated header and external-module comment. Compare the behavior noted for SFC 1.5.3, then clarify whether this input should be rejected or accepted with an empty output; done requires an agreed behavior and a regression check.

Written by the indexing model from the issue text.