Imports of validate command

PathSynopsis
flagPackage flag implements command-line flag parsing.
github.com/getkin/kin-openapi/openapi2
github.com/getkin/kin-openapi/openapi3
github.com/oasdiff/yaml
logPackage log implements a simple logging package.
osPackage os provides a platform-independent interface to operating system functionality.
stringsPackage strings implements simple functions to manipulate UTF-8 encoded strings.