[FIX] Do not find_package yourself

[eseiler]

CPM usually does a find_package redirect.
If find_package is called for some package added via CPM, it is redirected to CPM.

Before CPM 0.40.6, there was a bug that prevented this redirection from working properly.

If Seqan3 is added via CPM, CPM will call seqan3's CMakeLists.txt.
Because we use find_package there, it is redirected and results in seqan3 not being available.

[seqan-actions]

Documentation preview available at https://docs.seqan.de/preview/seqan/seqan3/3345

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.11%. Comparing base (d48a8c3) to head (26c5cbe).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3345      +/-   ##
==========================================
- Coverage   98.12%   98.11%   -0.01%     
==========================================
  Files         270      270              
  Lines       11915    11913       -2     
  Branches      103      103              
==========================================
- Hits        11691    11689       -2     
  Misses        224      224              

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