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/deepmap/oapi-codegen/cmd/oapi-codegen/oapi-codegen.go

@@ -0,0 +1,173 @@
+// Copyright 2019 DeepMap, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"github.com/deepmap/oapi-codegen/pkg/codegen"
+	"github.com/deepmap/oapi-codegen/pkg/util"
+)
+
+func errExit(format string, args ...interface{}) {
+	_, _ = fmt.Fprintf(os.Stderr, format, args...)
+	os.Exit(1)
+}
+
+func main() {
+	var (
+		packageName    string
+		generate       string
+		outputFile     string
+		includeTags    string
+		excludeTags    string
+		templatesDir   string
+		importMapping  string
+		excludeSchemas string
+	)
+	flag.StringVar(&packageName, "package", "", "The package name for generated code")
+	flag.StringVar(&generate, "generate", "types,client,server,spec",
+		`Comma-separated list of code to generate; valid options: "types", "client", "chi-server", "server", "spec", "skip-fmt", "skip-prune"`)
+	flag.StringVar(&outputFile, "o", "", "Where to output generated code, stdout is default")
+	flag.StringVar(&includeTags, "include-tags", "", "Only include operations with the given tags. Comma-separated list of tags.")
+	flag.StringVar(&excludeTags, "exclude-tags", "", "Exclude operations that are tagged with the given tags. Comma-separated list of tags.")
+	flag.StringVar(&templatesDir, "templates", "", "Path to directory containing user templates")
+	flag.StringVar(&importMapping, "import-mapping", "", "A dict from the external reference to golang package path")
+	flag.StringVar(&excludeSchemas, "exclude-schemas", "", "A comma separated list of schemas which must be excluded from generation")
+	flag.Parse()
+
+	if flag.NArg() < 1 {
+		fmt.Println("Please specify a path to a OpenAPI 3.0 spec file")
+		os.Exit(1)
+	}
+
+	// If the package name has not been specified, we will use the name of the
+	// swagger file.
+	if packageName == "" {
+		path := flag.Arg(0)
+		baseName := filepath.Base(path)
+		// Split the base name on '.' to get the first part of the file.
+		nameParts := strings.Split(baseName, ".")
+		packageName = codegen.ToCamelCase(nameParts[0])
+	}
+
+	opts := codegen.Options{}
+	for _, g := range splitCSVArg(generate) {
+		switch g {
+		case "client":
+			opts.GenerateClient = true
+		case "chi-server":
+			opts.GenerateChiServer = true
+		case "server":
+			opts.GenerateEchoServer = true
+		case "types":
+			opts.GenerateTypes = true
+		case "spec":
+			opts.EmbedSpec = true
+		case "skip-fmt":
+			opts.SkipFmt = true
+		case "skip-prune":
+			opts.SkipPrune = true
+		default:
+			fmt.Printf("unknown generate option %s\n", g)
+			flag.PrintDefaults()
+			os.Exit(1)
+		}
+	}
+
+	opts.IncludeTags = splitCSVArg(includeTags)
+	opts.ExcludeTags = splitCSVArg(excludeTags)
+	opts.ExcludeSchemas = splitCSVArg(excludeSchemas)
+
+	if opts.GenerateEchoServer && opts.GenerateChiServer {
+		errExit("can not specify both server and chi-server targets simultaneously")
+	}
+
+	swagger, err := util.LoadSwagger(flag.Arg(0))
+	if err != nil {
+		errExit("error loading swagger spec\n: %s", err)
+	}
+
+	templates, err := loadTemplateOverrides(templatesDir)
+	if err != nil {
+		errExit("error loading template overrides: %s\n", err)
+	}
+	opts.UserTemplates = templates
+
+	if len(importMapping) > 0 {
+		opts.ImportMapping, err = util.ParseCommandlineMap(importMapping)
+		if err != nil {
+			errExit("error parsing import-mapping: %s\n", err)
+		}
+	}
+
+	code, err := codegen.Generate(swagger, packageName, opts)
+	if err != nil {
+		errExit("error generating code: %s\n", err)
+	}
+
+	if outputFile != "" {
+		err = ioutil.WriteFile(outputFile, []byte(code), 0644)
+		if err != nil {
+			errExit("error writing generated code to file: %s", err)
+		}
+	} else {
+		fmt.Println(code)
+	}
+}
+
+func splitCSVArg(input string) []string {
+	input = strings.TrimSpace(input)
+	if len(input) == 0 {
+		return nil
+	}
+	splitInput := strings.Split(input, ",")
+	args := make([]string, 0, len(splitInput))
+	for _, s := range splitInput {
+		s = strings.TrimSpace(s)
+		if len(s) > 0 {
+			args = append(args, s)
+		}
+	}
+	return args
+}
+
+func loadTemplateOverrides(templatesDir string) (map[string]string, error) {
+	var templates = make(map[string]string)
+
+	if templatesDir == "" {
+		return templates, nil
+	}
+
+	files, err := ioutil.ReadDir(templatesDir)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, f := range files {
+		data, err := ioutil.ReadFile(path.Join(templatesDir, f.Name()))
+		if err != nil {
+			return nil, err
+		}
+		templates[f.Name()] = string(data)
+	}
+
+	return templates, nil
+}