Subset derived variable registry only with used derived variables

[aulemahal]

Change Summary

When searching for variables, the subset of the derived variable registry is constructed from the dependent variables that returned results when their query was searched.

This mainly changes two things:

• If a derived variable has a dependent that also exists as a derived variable in the DVR, that second dv is nonetheless not included in the subset. Fixing Don't include dependents when filtering the derived variable registry #445.
• If a requested variable corresponds to a derived variable but none of the dependents were found in the catalog, that derived variable is not included in the subset. I don't think this is a breaking change. That behaviour was not causing problems on my side, but I do think it's a bit cleaner this way?

Related issue number

Fixes #445.

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable

[andersy005]

@aulemahal, Thank you for a thorough report in #445 and for your work on this! I appreciate it!

This looks great to me 👍🏽