Avoid providing include directory as $prefix/include/IRT

**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 ` just works. That's not ideal since it leads to files that depends on this behavior instead of `#include `.

**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.