go: vendor the oapi-codegen cmd

See the comment in tools.go, I cannot fully explain what's happening here.
Somehow, Go 1.14 wants to use the vendored version of oapi-codegen but
without this file, oapi-codegen isn't vendored so the generation fails.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

vendor/github.com/getkin/kin-openapi/openapi3/refs.go

@@ -0,0 +1,199 @@
+package openapi3
+
+import (
+	"context"
+
+	"github.com/getkin/kin-openapi/jsoninfo"
+)
+
+type CallbackRef struct {
+	Ref   string
+	Value *Callback
+}
+
+func (value *CallbackRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *CallbackRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *CallbackRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type ExampleRef struct {
+	Ref   string
+	Value *Example
+}
+
+func (value *ExampleRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *ExampleRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *ExampleRef) Validate(c context.Context) error {
+	return nil
+}
+
+type HeaderRef struct {
+	Ref   string
+	Value *Header
+}
+
+func (value *HeaderRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *HeaderRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *HeaderRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type LinkRef struct {
+	Ref   string
+	Value *Link
+}
+
+func (value *LinkRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *LinkRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *LinkRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type ParameterRef struct {
+	Ref   string
+	Value *Parameter
+}
+
+func (value *ParameterRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *ParameterRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *ParameterRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type ResponseRef struct {
+	Ref   string
+	Value *Response
+}
+
+func (value *ResponseRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *ResponseRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *ResponseRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type RequestBodyRef struct {
+	Ref   string
+	Value *RequestBody
+}
+
+func (value *RequestBodyRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *RequestBodyRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *RequestBodyRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type SchemaRef struct {
+	Ref   string
+	Value *Schema
+}
+
+func NewSchemaRef(ref string, value *Schema) *SchemaRef {
+	return &SchemaRef{
+		Ref:   ref,
+		Value: value,
+	}
+}
+
+func (value *SchemaRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *SchemaRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *SchemaRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}
+
+type SecuritySchemeRef struct {
+	Ref   string
+	Value *SecurityScheme
+}
+
+func (value *SecuritySchemeRef) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalRef(value.Ref, value.Value)
+}
+
+func (value *SecuritySchemeRef) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalRef(data, &value.Ref, &value.Value)
+}
+
+func (value *SecuritySchemeRef) Validate(c context.Context) error {
+	v := value.Value
+	if v == nil {
+		return foundUnresolvedRef(value.Ref)
+	}
+	return v.Validate(c)
+}