Handle relative ext/ paths and test the windows installer

[bwoebi]

Description

Fixes src/ dir copy and avoids the .dll suffix at all costs for windows.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

Merging #2559 (17bbafb) into master (f5c4106) will decrease coverage by 27.78%.
Report is 2 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #2559       +/-   ##
=============================================
- Coverage     75.91%   48.13%   -27.78%     
  Complexity     2574     2574               
=============================================
  Files           240      214       -26     
  Lines         27029    23057     -3972     
  Branches        976        0      -976     
=============================================
- Hits          20519    11099     -9420     
- Misses         5990    11958     +5968     
+ Partials        520        0      -520     

Flag	Coverage Δ
appsec-extension	?
tracer-extension	78.70% <ø> (ø)
tracer-php	10.31% <ø> (-64.77%)	⬇️

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see 90 files with indirect coverage changes

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