Rename some variables to have a more representative name

[pepicrft]

Resolves #411

Short description 📝

As @gubikmic pointed out here, unsync is quite a vague and ambiguous prefix that makes it hard to understand what certain variables are for.

Solution 📦

Since variables prefixed by unsync- are not-thread-safe variables, I change the prefix to notThreadSafe- instead.

[yonaskolb]

How about just _projects?

[codecov]

Codecov Report

Merging #415 into master will decrease coverage by 0.01%.
The diff coverage is 68.18%.

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   78.83%   78.82%   -0.02%     
==========================================
  Files         140      140              
  Lines        6772     6771       -1     
==========================================
- Hits         5339     5337       -2     
- Misses       1433     1434       +1

Impacted Files	Coverage Δ
Sources/xcodeproj/Objects/Project/PBXObjects.swift	74.03% <68.18%> (ø)	⬆️
Sources/xcodeproj/Objects/Files/PBXGroup.swift	80.23% <0%> (-1.38%)	⬇️

---

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[codecov]

Codecov Report

Merging #415 into master will not change coverage.
The diff coverage is 78.35%.

@@           Coverage Diff           @@
##           master     #415   +/-   ##
=======================================
  Coverage   78.83%   78.83%           
=======================================
  Files         140      140           
  Lines        6772     6772           
=======================================
  Hits         5339     5339           
  Misses       1433     1433

Impacted Files	Coverage Δ
Sources/xcodeproj/Objects/Targets/PBXTarget.swift	74.76% <100%> (ø)	⬆️
...codeprojTests/Objects/Targets/PBXTargetTests.swift	100% <100%> (ø)	⬆️
Sources/xcodeproj/Objects/Project/PBXObjects.swift	74.03% <68.18%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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