fix: variable datasources shown as missing and unused datasource

[soonann]

This PR fixes a bug where datasources that were being used in dashboards through variables were being wrongly categorised as unused (in explore datasources) and missing (in explore dashboards).

The full description of the problem can be seen in the issue #164 along with steps to reproduce it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.20%. Comparing base (f756410) to head (12294d4).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   86.20%   86.20%           
=======================================
  Files           9        9           
  Lines        1109     1109           
=======================================
  Hits          956      956           
  Misses        153      153           

Flag	Coverage Δ
unittests	86.20% <100.00%> (ø)

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[amotl]

Thank you very much for reporting and submitting a fix for GH-164.