Avoid providing include directory as $prefix/include/IRT

[wdconinc]

Is your feature request related to a problem? Please describe.
The CMake-installed config file exposes $prefix/include/IRT as the include path. This means that e.g. #include <CherenkovDetector.h> just works. That's not ideal since it leads to files that depends on this behavior instead of #include <IRT/CherenkovDetector.h>.

Describe the solution you'd like
The CMake-installed config should expose $prefix/include as the include path and require downstream code to include explicitly with the IRT/ directory prefix.

Describe alternatives you've considered
I understand this may be a breaking change, depending on how much of the downstream code relies on this.