Use CMake imported targets to link Poco

[traversaro]

The use of CMake imported targets to link Poco has the following advantages:

• It generated relocatable cmake config files (see https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#creating-relocatable-packages)

• It does not raise an error like:

CMake Error in build/_deps/abb_librws-src/CMakeLists.txt:
  Target "abb_librws" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "C:/Users/gcorrenti/idjl-software/.pixi/envs/idjllancedeformationenv/Library/include"

  which is prefixed in the source directory.

when the abb_librws is installed in a folder that is contained in source folder of the cmake project that is calling find_package(abb_librws), as it commonly happens if Poco is installed in a project-oriented package manager, for example pixi or vcpkg when used in manifest mode.

I checked in Ubuntu and Poco imported targets are available in Poco's apt packages at least since Ubuntu 18.04, so I think we should be safe w.r.t. to backward compatibility.

[traversaro]

fyi @Luca0101