Extend AUTODOC_FormatDate to deal with strings, use it to parse Date in PackageInfo.g

[fingolfin]

... as used in the Date field of PackageInfo.g files.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.72%. Comparing base (88e8e52) to head (471aeb1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/ToolFunctions.gi	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
+ Coverage   81.64%   81.72%   +0.07%     
==========================================
  Files          13       13              
  Lines        2158     2161       +3     
==========================================
+ Hits         1762     1766       +4     
+ Misses        396      395       -1     

Files with missing lines	Coverage Δ
gap/AutoDocMainFunction.gi	81.08% <100.00%> (-0.15%)	⬇️
gap/ToolFunctions.gi	93.10% <93.75%> (+0.05%)	⬆️

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