convert: what to do with `fn` and archive source files

[lucascolley]

Following #48, invalid recipes can be produced due to the warning at https://rattler-build.prefix.dev/latest/reference/recipe_file/#specifying-a-file-name. This was briefly discussed in https://discord.com/channels/1082332781146800168/1483742267641430171 and the verdict seemed to be that it would be good if convert avoided producing file_name for archive source files, perhaps instead omitting it and issuing a warning (or just rejecting the input and advising to remove fn?)

Maybe it can be clever and rename things in the rest of the recipe, but I think just making people aware that there is no easy direct translation would suffice.

[schuylermartin45]

Honestly I'm not up on this discussion. The fn field conversion hasn't probably been touched in a while now. We definitely have other types of warnings for similar difficult scenarios, so I'm not opposed to changing this up.

So long as this change is agreeable to the majority of the Conda Community. It doesn't immediately sound conda-forge specific, as far as I can tell.