Prepare the candidate before calculating the hash

[frostming]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Related to #1103

[codecov-commenter]

Codecov Report

Merging #1108 (4572e90) into dev (a0fa365) will decrease coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##              dev    #1108      +/-   ##
==========================================
- Coverage   83.92%   83.88%   -0.05%     
==========================================
  Files          79       79              
  Lines        6911     6918       +7     
  Branches     1638     1639       +1     
==========================================
+ Hits         5800     5803       +3     
- Misses        762      764       +2     
- Partials      349      351       +2     

Flag	Coverage Δ
unittests	83.65% <75.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
pdm/models/repositories.py	76.68% <75.00%> (-0.18%)	⬇️
pdm/models/candidates.py	87.63% <0.00%> (-0.73%)	⬇️

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