(feature): Add Go snippet generation

[amckinney]

This introduces Go snippet generation for any API definition that defines examples, such as the following:

import (
  context "context"
  time "time"
  acme "github.com/acme/acme-go"
  acmeclient "github.com/acme/acme-go/client"
  option "github.com/acme/acme-go/option"
)

client := acmeclient.NewClient(
  option.WithToken(
    "<YOUR_AUTH_TOKEN>",
  ),
)
response, err := client.User.Create(
  context.TODO(),
  &acme.CreateUserRequest{
    Username: "john",
    Language: acme.LanguageEnglish,
  },
)

Given that this was such a substantial feature, this PR also moved the Go generator's seed tests into the top-level seed directory so that all of the generated Go fixtures exist alongside the rest of the generators.

This also resolved a small bug that caused package documentation to not be generated into the correct doc.go file, which sometimes resulted in non-deterministic generation results (i.e. the last doc.go won).

[dsinghvi]

Thanks for moving the seed snapshots to the root