easier preloaded plugin building

The current docs at https://github.com/ipfs/go-ipfs/blob/master/docs/plugins.md#preloaded-plugins show that all you need to do is modify the `plugin/loader/preload_list` and then run `make build`.

Currently this will error first telling you to run `go get mypluginrepo` and then after it'll error telling you the `go.sum` file is out of date and you'll have to run `go mod tidy`.

This should take fewer steps to make it easier for people less familiar with Go to include new plugins.