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/pkg/util/loader.go

@@ -0,0 +1,20 @@
+package util
+
+import (
+	"net/url"
+
+	"github.com/getkin/kin-openapi/openapi3"
+)
+
+func LoadSwagger(filePath string) (swagger *openapi3.Swagger, err error) {
+
+	loader := openapi3.NewSwaggerLoader()
+	loader.IsExternalRefsAllowed = true
+
+	u, err := url.Parse(filePath)
+	if err == nil && u.Scheme != "" && u.Host != "" {
+		return loader.LoadSwaggerFromURI(u)
+	} else {
+		return loader.LoadSwaggerFromFile(filePath)
+	}
+}