Now that we're on Go 1.16+ we can do this easily! The main advantage is
it means users can build a genqlient binary and use that portably (or we
could distribute one, or whatever). Plus the code is marginally simpler;
the `embed` API is really quite nice.
Fixes#9.
Test plan:
```
make check
go build .
rm -rf generate # pretend we have no checkout
./genqlient ./internal/integration/genqlient.yaml
./genqlient --init # fails after generating a default config
```
