Move graph related functions in image to graph package.
Consolidating graph functionality is the first step in refactoring graph into an image store model.
Subsequent refactors will involve breaking up graph into multiple types with a strongly defined interface.
Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Upstream-commit: 2b58b677a54950bc78c13760ef79fe6284154847
Component: engine
6825ac1c62