Fails when built with -trimpath

### What happened?

```
$ GOFLAGS='-trimpath' go build github.com/99designs/gqlgen && ./gqlgen
modelgen: locating templates: lstat github.com/99designs/gqlgen@v0.10.1/plugin/modelgen: no such file or directory
[EXIT:3] $
```

### What did you expect?

Success, like without `-trimpath`:

```
$ GOFLAGS=' ' go build github.com/99designs/gqlgen && ./gqlgen
$
```

### Minimal graphql.schema and models to reproduce

Just about anything, really.

### versions
- `gqlgen version`?
- `go version`?
- dep or go modules?

```
$ go list -m github.com/99designs/gqlgen
github.com/99designs/gqlgen v0.10.1
$ go version
go version go1.13.3 linux/amd64
```

And obviously modules.

It seems gqlgen is using build-time paths to try to locate its template files, and trimpath confuses that. Perhaps the templates should be bundled into the executable.