Disallow user from importing several Arend modules at once.

[sxhya]

In the attached gif you can see that importing a directory with 2 Arend modules is allowed and that no errors are reported.

I propose to do the following.

Import Arend module feature should recursively scan for arend.yaml files in the folder tree. If no YAML-files are found, warning should be given to the user. If multiple YAML files are found, 1 Arend module should be created per each Yaml. The situation mentioned in the video should not happen.

[valis]

There should be just 1 yaml file in the root. No scanning is required.