kramdown-rfc-extract-sourcecode: One name= and multiple artset alternatives

[cabo]

Providing an artwork-name/sourcecode-name on something that will be an artset (e.g., artwork marked aasvg) gives the same name to the alternatives. These are put separately into the right directory each, but all with the same name. This causes slight miscomfort in https://github.com/ietf-wg-scitt/draft-ietf-scitt-architecture/pull/458, where the SVGs that aasvg generates are extracted with the name suffix .ascii-art. This is a limitation of kramdown-rfc-extract-sourcecode that we'll need to address.