Fix issue when ip_link() is called on IP defined in different src dir

[Risto97]

There is a quirk with INTERFACE_LINK_LIBRARIES property described here: https://cmake.org/cmake/help/v3.30/prop_tgt/INTERFACE_LINK_LIBRARIES.html

When an IP is defined in a subdirectory.
If in another SOURCE directory than where IP was defined we do ip_link() linking another IP to that IP, CMake adds a wrapper around INTERFACE_LINK_LIBRARIES, like this:
::@(directory-id);...;::@

Graph traversal function should filter these directory-id elements.
It is safer than modifying INTERFACE_LINK_LIBRARIES itself upon ip_link process, as it is not clear how this mechanism is useful for other generators.

[Risto97]

resolve #39