Ensure embedded spec contains all necessary OpenAPI documents

When using a schema that uses `$ref`s to an external file, although we
generate the mappings appropriately, this then leads to the embedded
spec being misaligned, as it continues to use the `$ref` to that
external file, which then is not available when calling i.e.
`GetSwagger`.

We can take advantage of the `InternalizeRefs` method to perform an
in-place modification to apply those external references into a single,
embeddable spec.

This appears to only work when a full file is referenced, instead of a
path within a file.

internal/test/externalref/object_c.json

@@ -0,0 +1,5 @@
+{
+  "type": "object",
+  "properties": {},
+  "additionalProperties": true
+}

internal/test/externalref/spec.yaml

@@ -9,3 +9,5 @@ components:
           $ref: ./packageA/spec.yaml#/components/schemas/ObjectA
         object_b:
           $ref: ./packageB/spec.yaml#/components/schemas/ObjectB
+        object_c:
+          $ref: ./object_c.json

pkg/codegen/inline.go

@@ -16,6 +16,7 @@ package codegen
 import (
 	"bytes"
 	"compress/gzip"
+	"context"
 	"encoding/base64"
 	"fmt"
 	"text/template"
@@ -26,6 +27,8 @@ import (
 // This generates a gzipped, base64 encoded JSON representation of the
 // swagger definition, which we embed inside the generated code.
 func GenerateInlinedSpec(t *template.Template, importMapping importMap, swagger *openapi3.T) (string, error) {
+	// ensure that any external file references are embedded into the embedded spec
+	swagger.InternalizeRefs(context.Background(), nil)
 	// Marshal to json
 	encoded, err := swagger.MarshalJSON()
 	if err != nil {