Resource inheritance fixes

[bkoelman]

To render selectors for derived types, a selector for each non-abstract derived type must be available in QueryLayer.Selection, which wasn't the case.

Parsing of include chains is changed to include relationships on all concrete derived types.
For example, given an inclusion chain of "wheels" at /vehicles, the parser now returns Car.Wheels and Truck.Wheels. This used to be Vehicle.Wheels. There's no point in adding abstract types, because they can't be instantiated in selectors.

QueryLayerComposer was fixed to generate selectors for the left-type of these relationships, instead of for the QueryLayer resource type (so Car.Wheels and Truck.Wheels instead of Vehicle.Wheels).

SelectClauseBuilder was missing a cast to the derived type when descending into relationship selectors, which is fixed now.

Being unable to include relationships of sibling types after a POST/PATCH resource request at a base endpoint was because a QueryLayer for fetch-after-post was built against the accurized resource type, and then sent to the original (non-accurized) repository. For example, POST /vehicles?include=TruckRelationship,CarRelationship only works if the query executes against the non-accurized table (so Vehicle instead of Car, because Car doesn't contain TruckRelationship). The fix is to discard the accurized resource type and use the original TResource.

Other improvements:

• During the process of building expression trees, consistency checks have been added to prevent downstream crashes that are difficult to diagnose.
• SelectClauseBuilder internally passed along a LambdaScope that overruled the one present in context, so care had to be taken to use the right one. To eliminate this pitfall, now a new context is forked which contains the appropriate LambdaScope, so the overruling parameter is no longer needed.

Fixes #1639, fixes #1640.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 9 lines in your changes missing coverage. Please review.

Project coverage is 90.68%. Comparing base (55f671c) to head (cfe159b).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...ies/QueryableBuilding/QueryClauseBuilderContext.cs	50.00%	2 Missing and 1 partial ⚠️
...Core/Queries/QueryableBuilding/QueryableBuilder.cs	40.00%	2 Missing and 1 partial ⚠️
...e/Queries/QueryableBuilding/SelectClauseBuilder.cs	90.32%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1641      +/-   ##
==========================================
- Coverage   90.73%   90.68%   -0.06%     
==========================================
  Files         353      353              
  Lines       11492    11514      +22     
  Branches     1888     1897       +9     
==========================================
+ Hits        10427    10441      +14     
- Misses        696      702       +6     
- Partials      369      371       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.