/nextclade: Missing dataset

https://nextstrain.org/nextclade only shows datasets from the "nextstrain" and "community" collections. It's missing datasets from other collections. Currently there is only one `enpen/enterovirus/ev-d68`, but I suspect there will be more in the future. ### Possible solutions 1. Special-case `enpen` and other future collections similar to existing special-casing of `community`. PR: #1302 2. Add the `nextstrain` prefix, at least internally, to allow for simpler code like dc9fd677. This would also align better with Nextclade itself which keeps the prefix [externally](https://docs.nextstrain.org/projects/nextclade/en/latest/user/datasets.html) and presumably internally.