* Allow absolute paths in schema files.
Thsi is useful for some out-of-tree testing I want to do. It's
unfortunate (imo) that filepath.join doesn't have this behavior by
default.
Test plan:
go test ./...
* run gofmt
* one more change to use pathJoin
Code is mostly borrowed from [gqlgen](https://github.com/99designs/gqlgen).
The idea here is that I want to be able to store `genqlient.yaml` at the top-level, but my client code lives down in `graph/client/`. I put the `//go:generate` line in `graph/client/client.go`.