[Rubygems 4 compatibility] Do not extract files if destination directory exists

[jenshenny]

CI is failing on RubyGems 4 https://github.com/rubygems/rubygems.org/actions/runs/19900740275/job/57046017773?pr=6125

GemPackageEnumeratorTest#test_: #map should enumerate all the same files as Gem::Package#contents. :
Errno::EEXIST: File exists @ syserr_fail2_in - /home/runner/work/rubygems.org/rubygems.org/tmp/gems/bin_and_img-0.1.0/exe/hello
    test/unit/gem_package_enumerator_test.rb:9:in 'block in <class:GemPackageEnumeratorTest>'

We shouldn't extract files if it's been extracted already.

Relevant RG change: ruby/rubygems#8974

File structure

tmp/gems/bin_and_img-0.1.0/
  exe/hello
  lib/bin_and_img.rb
  lib/bin_and_img/version.rb
  images/ruby.png

The clause in GemPackageEnumeratorTest is incorrect since it is checking tmp/gems/bin_and_img-0.1.0/bin_and_img-0.1.0 and not tmp/gems/bin_and_img-0.1.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (d447b16) to head (35fa5cf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6126   +/-   ##
=======================================
  Coverage   97.24%   97.24%           
=======================================
  Files         476      476           
  Lines        9785     9785           
=======================================
  Hits         9515     9515           
  Misses        270      270           

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