[vcpkg] Make information about dependent ports available to currently building port

[Neumann-A]

This is required to correctly detect/transport/setup transitiv dependencies in the current port if it is port which is not build with CMake like Qt.

The following variables would be good: (see also #8239 (comment))

${PORT}_DEPENDENT: Linear unique list of *all* dependent ports
${PORT}_${DEPENDENT}_FEATURES: List of features <dependent> has been built with
${PORT}_${DEPENDENT}_DEPENDENT: List of dependent ports of <dependent>

[Neumann-A]

maybe even better would be

PORTS_INSTALLED: Linear list of *all* installed ports
${INSTALLED}_FEATURES: List of features <installed> has been built with
${INSTALLED}_DEPENDENT: List of dependent ports of <installed>

because it automatically includes the above and gives even more information

and theoretically vcpkg could also provide/set the variables

${INSTALLED}_LIBRARIES_RELEASE: list of all libraries installed into ${CURRENT_INSTALLED_DIR}/lib by <installed>
${INSTALLED}_LIBRARIES_DEBUG: fullpath of all libraries installed into ${CURRENT_INSTALLED_DIR}/debug/lib by <installed>

since vcpkg exactly knows which libraries are being installed into the lib and debug/lib folder. (which means it is not necessary to call find_library from portfiles)