Avoid Creating Alias Target If It Already Exists

[threeal]

This issue addresses the lines of code that create an alias target for a specific namespace: https://github.com/TheLartians/PackageProject.cmake/blob/2d8a4685857c768da7ad1d8a7c6e07b1b5285cca/CMakeLists.txt#L33-L42

It is suggested to create the alias target only if the alias does not already exist; otherwise, it may cause issues as highlighted in https://github.com/boost-ext/ut/pull/564.

Before CMake 3.18, this may not have caused an issue because any new alias target would replace the old alias. However, the behavior has changed since then (see also).