CMake: Don't attempt to create alias target if target of the same name already exists

[dweindl]

Fixes #2546.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.79%. Comparing base (d4bb0ce) to head (aaf69bc).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2547   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files          325      325           
  Lines        21058    21058           
  Branches      1473     1473           
========================================
  Hits         16383    16383           
  Misses        4648     4648           
  Partials        27       27           

Flag	Coverage Δ
cpp	73.51% <ø> (ø)
cpp_python	34.05% <ø> (ø)
petab	37.05% <ø> (ø)
python	72.34% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.