Improve handling of auto-mount path collisions

[amisevsk]

Description

When mounting configmaps/secrets as files to a workspace, DWO currently doesn't detect path collisions (e.g. mounting two files via subpath to the same path). When this occurs, container creation will fail with a message that doesn't make the issue clear, and the DevWorkspace will fail due to a CreateContainerError event.

Instead, DWO should try to detect these issues before they occur and fail the workspace with an appropriate message.

Additional context