Add an attribute to hw.module.extern referencing emit.file ops that implement them

[tmckay-sifive]

In order to generate accurate file lists for a subset of the circuit, we must know what files correspond to extmodules if they are emitted by firtool (for example, FIRRTL extmodules with a BlackBoxInlineAnno).

This change adds an optional attribute files to the hw.module.extern op which refers to any emit.file ops corresponding to it.

[fabianschuiki]

If I understand correctly, this makes hw.module.extern refer to an emit.file, and emit.file already refers to hw.module.extern. Do we want circular symbol/name references in the IR? Since the emit.file op and its nested ops already refer to hw.module.extern ops by their symbol, that should give us the extmodule -> file association that you're looking for.

(This probably makes generating file lists easier because there's less IR traversal that needs to happen in that tool. But I'm wondering if we shouldn't put the complexity of this reverse lookup into that tool instead of making the IR more complex.)