It's not fully wired in, `convertQuery` for Arxiv is implemented as
`undefined`.

This commit simplifies the `ExternalAPIs` type, which is now a pure
enumeration, moving the lookup of the Pubmed key at the usage sites.
This simplified the code quite a bit, and made some otherwise-effectful
functions pure.

NOTE This doesn't compile yet.

This still isn't optimal because in our code we use lists, so this
will fetch everything (using conduit), coerce into a list and then
process.

This turned out to be quite heavy because of module import cycles.