checklayer: Skip layers without a collection

As in other places in the file, skip layers that don't define a
collection when searching for a layer to resolve a dependency. Fixes
KeyError exceptions when attempting to access the layer collections
later

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 26090a28)
Signed-off-by: Steve Sakoman <steve@sakoman.com>