In commit a1ad0a699b (target_link_libraries: Allow use with targets in
other directories, 2018-09-07, v3.13.0-rc1~94^2) we accidentally broke
the logic that adds `$<LINK_ONLY:...>` to private dependencies of static
libraries in their `INTERFACE_LINK_LIBRARIES` in the case that the
dependency is added from outside the directory creating the library.
The check for a valid target name should apply to the original name
specified by the caller and not the encoded cross-directory reference.
Fixes: #19197
3d3713121b