toaster: implement API to get full list of deps

toaster: implement API to get full list of deps

Implemented Layer_Version.get_alldeps API to recursively get
full list of dependencies for the layer. Dependencies that are
already in the project are filtered out from the result.
Result list of Layer_Version objects is sorted by layer name
for UI to look consistent.

This API is going to be used to show amount and list of
dependencies for the layer in the list of compatible layers
for the project.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>