Avoid variable_map in TypedSymbol.get_derived_type_member and verify type information is derived correctly

[reuterbal]

I played around a little with this because I wanted to see if we can get away without using the symbol's variable_map, which can be costly for large and nested derived types. For this, I've also added an additional test that validates that type information is derived correctly.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/285/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.88%. Comparing base (03170f5) to head (20496ab).

Additional details and impacted files

@@                Coverage Diff                @@
##           naan-stack-fixes     #285   +/-   ##
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  Coverage             92.88%   92.88%           
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  Files                   102      102           
  Lines                 18267    18266    -1     
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  Hits                  16967    16967           
+ Misses                 1300     1299    -1     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	92.85% <100.00%> (+<0.01%)	⬆️
transformations	92.24% <ø> (ø)

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[awnawab]

So the type information is looked in the Variable factory method via the parent? Very clever! many thanks for this!