Fix Products group serialisation with temporary ids

[damirdavletov]

Short description 📝

When adding remote xcodeproj bundle reference to the project, remote product references are serialised with temporary ids.

Solution 📦

Fix serialisation of Products group of the remote project references

[codecov]

Codecov Report

Merging #489 into master will increase coverage by 0.39%.
The diff coverage is 98.71%.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   82.46%   82.85%   +0.39%     
==========================================
  Files         148      149       +1     
  Lines        8073     8149      +76     
==========================================
+ Hits         6657     6752      +95     
+ Misses       1416     1397      -19

Impacted Files	Coverage Δ
...XcodeProjTests/Utils/ReferenceGeneratorTests.swift	100% <100%> (ø)
Sources/XcodeProj/Utils/ReferenceGenerator.swift	88.12% <95.45%> (+1.69%)	⬆️
...codeProj/Objects/Files/PBXContainerItemProxy.swift	86.04% <0%> (+1.16%)	⬆️
Sources/XcodeProj/Objects/Project/PBXObjects.swift	75.81% <0%> (+2.52%)	⬆️
Sources/XcodeProj/Objects/Project/PBXProj.swift	71.84% <0%> (+3.88%)	⬆️
.../XcodeProj/Objects/Targets/PBXReferenceProxy.swift	23.07% <0%> (+23.07%)	⬆️

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[damirdavletov]

Thanks @pepibumur !