Don't install gtest if building from source

[hcho3]

Also, enabling CPack.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.65 ⚠️

Comparison is base (2566b4d) 85.32% compared to head (4eb6522) 84.68%.

❗ Current head 4eb6522 differs from pull request most recent head d160ee9. Consider uploading reports for the commit d160ee9 to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##             mainline     #491      +/-   ##
==============================================
- Coverage       85.32%   84.68%   -0.65%     
  Complexity         42       42              
==============================================
  Files             112      111       -1     
  Lines            9910     8849    -1061     
  Branches          617       40     -577     
==============================================
- Hits             8456     7494     -962     
+ Misses           1431     1332      -99     
  Partials           23       23              

see 68 files with indirect coverage changes

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