Handle case when spec shortname is "package"

[tidoust]

Via #468 (comment)

While we don't have such a spec for the time being, a future spec could take the shortname package. If such a spec exists and defines CSS, elements, or any other content that ends up in a JSON extract to be included in an NPM package, we'll end up with a conflict because NPM packages already contain a package.json file.

The right solution is probably to move all extracts in NPM packages to a subfolder instead of at the root level.

A not-so-right solution would be to force a different shortname in such a case but then some projects may rely on file extracts being named after the real spec's shortname.

A very convenient approach would be to claim that this won't happen and defer resolution until we actually face the issue...