Using import "foo.yml" syntax expects the working directory to be "foo"

This is a strange and undocumented feature. It means you can't easily split sections of the yaml up by logical grouping. Instead, it expects sections to be by file system layout. For example, if you called the include file `tutorials.yml`, but actual things were scattered, we have to work around it with things like
```
tutorials.yml

tasks:
concept1:
working_directory: ../training/concept1
getting_started:
working_directory: ../basics/getting_started
```

The issue names YAML imports and the working_directory field but no source files or tests. Start by locating the YAML import handling and existing tests for import and working_directory behavior. Clarify whether imports should follow logical grouping or retain filesystem-relative behavior; done should include an agreed behavior and coverage or documentation.

Written by the indexing model from the issue text.