ERROR: the following packages/stacks could not have their rosdep keys resolved to system dependencies: zed_examples

For your case specifically, rosdep tried to find the information about zed_display_rviz and all other packages listed but couldn't find them. rosdep looks up to the file github.com/ros/rosdistro/blob/melodic/distribution.yaml for binary package in ROS Melodic, where there's no definition added for any of those packages. rosdep can also look up source packages, but you gave --ignore-src option, which lets rosdep ignore source packages, so it doesn't matter whether you have those packages by source or not. Without knowing what packages you have in your workspace, I'd say either not to pass --ignore-src, or have the source of all those failed packages in the workspace.