Introduce containerd-shim-runhcs-v1 on Windows

Implements the containerd-shim-runhcs-v1 shim on Windows for the runtime
v2 shim API.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>

vendor/github.com/Microsoft/hcsshim/internal/wclayer/layerexists.go

@@ -0,0 +1,25 @@
+package wclayer
+
+import (
+	"github.com/Microsoft/hcsshim/internal/hcserror"
+	"github.com/sirupsen/logrus"
+)
+
+// LayerExists will return true if a layer with the given id exists and is known
+// to the system.
+func LayerExists(path string) (bool, error) {
+	title := "hcsshim::LayerExists "
+	logrus.Debugf(title+"path %s", path)
+
+	// Call the procedure itself.
+	var exists uint32
+	err := layerExists(&stdDriverInfo, path, &exists)
+	if err != nil {
+		err = hcserror.Errorf(err, title, "path=%s", path)
+		logrus.Error(err)
+		return false, err
+	}
+
+	logrus.Debugf(title+"succeeded path=%s exists=%d", path, exists)
+	return exists != 0, nil
+}