* Add the toplevel module
So the go tooling stops throwing weird errors. Without that, running
make fails with the following message:
```
PATH="/…/.tools:${PATH}" go generate ./...
can't load package: package ./exporters/metric/dogstatsd: code in
directory /…/exporters/metric/dogstatsd expects import
"github.com/open-telemetry/opentelemetry-go-contrib/exporters/metric/dogstatsd"
can't load package: package ./plugins/sample/trace: code in directory
/…/plugins/sample/trace expects import
"github.com/open-telemetry/opentelemetry-go-contrib/plugins/sample/trace"
make: *** [Makefile:113: generate] Error 1
```
* Rename the module to use the vanity URL
The `go.opentelemetry.io/contrib` vanity URL is working now, so let's
officially switch to it.