(Federation) Models and executable schema can't exist in different packages

### What happened?

Trying out federation, the executable schema couldn't make use of the models if they were in a different package.

Using a config like:

```yaml
schema:
- schema.graphql
exec:
filename: generated/executableschema.go
model:
filename: models/models.go
resolver:
filename: resolver/resolver.go
type: Resolver
federated: true
autobind: []
```

It produced a model like

```go
type _Entity interface {
Is_Entity()
}
```

Which, not starting with a capital meant package generated couldn't use it from package models, with some of that code looking like

```go
res := resTmp.([]models._Entity) // <--- the issue
fc.Result = res
return ec.marshalN_Entity2ᚕgithubᚗcomᚋzerbitxᚋgapiᚋmodelsᚐ_Entity(ctx, field.Selections, res)
```

### What did you expect?

The Apollo federation extra bits to be added like service/_entities etc, and everything to generally work the same as before adding `federated: true`

### Minimal graphql.schema and models to reproduce

```graphql
type Whatevs @key(fields: "id") {
id: ID!
}
```

### versions
- `gqlgen version`?
v0.10.2-dev
- `go version`?
1.13.4
- dep or go modules?
modules

### Easy Workaround

Keep the model and exec in the same package

### Thanks

Thank you @marwan-at-work I'm very grateful for your efforts here.