Refactor course-section references to use compound keys

[n-tennyson]

Summary

This PR updates the course-section relationship in api-tools to use compound keys instead of partial references.

Changes

• Courses now store sections using full SectionKey
• Sections reference courses using CourseKey
• Fixed a bug in validator.go where invalid references could be skipped
• Updated parser tests and fixtures to match the new structure

Notes

• All tests pass (go test ./...)
• This only covers the course/section refactor
• Professor-section changes are handled separately (ProfSectionKey not modified)

[KBui4]

I haven't looked into this in too much detail, but I did notice that the TestSectionReferenceProfFail case was removed for seemingly no reason. I don't think that test should be deleted, but if it is causing issues, you could comment it out for now.