Fix: Use constants

[localheinz]

This PR

• uses private constants instead of private, static fields

Follows #425 (comment).

/cc @Tobion

[codecov]

Codecov Report

Merging #427 (2cb22d4) into main (f80e560) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##                main      #427   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       164       164           
===========================================
  Files             34        34           
  Lines            512       512           
===========================================
  Hits             512       512           

Impacted Files	Coverage Δ	Complexity Δ
src/Vendor/Composer/ConfigHashNormalizer.php	100.00% <100.00%> (ø)	8.00 <0.00> (ø)
src/Vendor/Composer/PackageHashNormalizer.php	100.00% <100.00%> (ø)	7.00 <0.00> (ø)
...rc/Vendor/Composer/VersionConstraintNormalizer.php	100.00% <100.00%> (ø)	7.00 <0.00> (ø)

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