The entries under the new `package_bindings` field should be packages,
but it's an easy mistake to put a file path instead (most of the other
fields in `genqlient.yaml` are files). Due to some bizzare behavior from
`go/packages` (described in #220), if you do that you get weird broken
code that gives you no clue what is wrong. Instead, let's guess if what
you gave us looks like a filename, and report a nice error if so.
Test plan: crossed fingers