feat: allow imports from same generated package

[avinassh]

This is a small change, but it lets you import marshaler and unmarshalers from the same generated package.

Why? I'd like to keep all the graphql related code into one. Following the conventions of the genqlient/example, I would like to keep a custom marshaler in the same directory as example. For e.g., example/helper.go could contain:

// helper.go
func UnMarshalDateTime(b []byte, v *time.Time) error {
    ...
    return nil
}

But following won't work:

...
generated: generated.go
package: main
bindings:
  DateTime:
    type: time.Time
    unmarshaler: "https://github.com/Khan/genqlient/main.UnMarshalDateTime" // since package name is main
    # unmarshaler: "https://github.com/Khan/genqlient/example.UnMarshalDateTime" // nor this, if one is using a custom package

So, instead I would like to add a new feature where one can import from the same package:

...
generated: generated.go
package: main
bindings:
  DateTime:
    type: time.Time
    unmarshaler: "main.UnMarshalDateTime"

What do you think? If you think this is good, then I will add tests and update the documentation

I have:

• [x] Written a clear PR title and description (above)
• [x] Signed the Khan Academy CLA
• [ ] Added tests covering my changes, if applicable
• [ ] Included a link to the issue fixed, if applicable
• [ ] Included documentation, for new features
• [ ] Added an entry to the changelog

[benjaminjkraft]

Closing in favor of #316,

[avinassh]

Hey @benjaminjkraft I missed the previous notification. I checked #316 and I think its in the right direction 👍