Re-vendor, and use mtrmac/image-spec:id-based-loader to fix tests

Anyone running (vndr) currently ends up with failing tests in OCI schema
validation because gojsonschema has fixed its "$ref" interpretation, exposing
inconsistent URI usage inside image-spec/schema.

So, this runs (vndr), and uses mtrmac/image-spec:id-based-loader
( https://github.com/opencontainers/image-spec/pull/739 ) to make the tests pass
again.  As soon as that PR is merged we should revert to using the upstream
image-spec repo again.

vendor/github.com/opencontainers/runc/libcontainer/user/lookup_unsupported.go

@@ -1,38 +0,0 @@
-// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris
-
-package user
-
-import (
-	"io"
-	"syscall"
-)
-
-func GetPasswdPath() (string, error) {
-	return "", ErrUnsupported
-}
-
-func GetPasswd() (io.ReadCloser, error) {
-	return nil, ErrUnsupported
-}
-
-func GetGroupPath() (string, error) {
-	return "", ErrUnsupported
-}
-
-func GetGroup() (io.ReadCloser, error) {
-	return nil, ErrUnsupported
-}
-
-// CurrentUser looks up the current user by their user id in /etc/passwd. If the
-// user cannot be found (or there is no /etc/passwd file on the filesystem),
-// then CurrentUser returns an error.
-func CurrentUser() (User, error) {
-	return LookupUid(syscall.Getuid())
-}
-
-// CurrentGroup looks up the current user's group by their primary group id's
-// entry in /etc/passwd. If the group cannot be found (or there is no
-// /etc/group file on the filesystem), then CurrentGroup returns an error.
-func CurrentGroup() (Group, error) {
-	return LookupGid(syscall.Getgid())
-}