Headers installation location

Use CamelCase headers in this module's own code

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Simplify headers installation location

See teams/frameworks-devs/kf6-workboard#3 (comment 431683)

For PrisonScanner, we can go with the CamelCase include dir, since it hasn't been in a KF release yet, so no backwards compatibility.

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Replicate the include dir layout in the builddir

So that e.g. #include <Prison/Prison> works at build time, the same way it should work when other modules include it.

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