NOJIRA: Made the intention of the reactor code more clear.

[MartinKanters]

This PR used to be part of MNG-7390, but now just contains the refactoring part.

Made the intention of the reactor code more clear and changed the order of steps to minimize sorting calls.
Changes:

• The main "reactor" method (DefaultGraphBuilder#reactorDependencyGraph) is more descriptive.. that one method should describe exactly every business logic rule, instead of having hidden business rules inside methods. In total I think it's easier to understand to full flow now.
• It used to sort the intermediate project list too many times
• It used to calculate and add the also-make/also-make-dependent projects into the intermediate project list too many times.
• The methods used for getting and trimming the intermediate project list were made consistent. They now additionally:
  • fail fast when it's not applicable (i.e. skip over getResumedProjects when -rf is not given),
  • return Sets, as the project lists do not need to be sorted in between and unique elements are automatically guaranteed,
  • are converted a bit more to Streams for the sake of readability.

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