Refactored SealedToSealed rule, re-enabled coproduct ambiguity check n Scala 3, fixed errors with too conservative checks

[MateuszKubuszok]

• split SealedToSealedRule body into smaller methods
• fix overconservative check (it used to check that all From subtypes names are unique, and all To subtypes names are unique when only non-overriden-manually From types should have unambigous To names matches)
• re-enable check for that on Scala 3
• await fixing Incorrect children queried before class was discovered error when typeCheckErrors is used scala/scala3#18484

[codecov]

Codecov Report

Patch coverage: 65.04% and project coverage change: +0.07% 🎉

Comparison is base (3ed1415) 79.04% compared to head (9957cb1) 79.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   79.04%   79.11%   +0.07%     
==========================================
  Files         105      105              
  Lines        3960     3974      +14     
  Branches      178      124      -54     
==========================================
+ Hits         3130     3144      +14     
  Misses        830      830              

Files Changed	Coverage Δ
...rmSealedHierarchyToSealedHierarchyRuleModule.scala	67.25% <65.04%> (+7.07%)	⬆️

... and 10 files with indirect coverage changes

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[MateuszKubuszok]

Blocked by a bug in scala.compiletime.testing.typeCheckErrors, reproduced in https://gist.github.com/MateuszKubuszok/2723c2842b8bf969618f4bd751ba3041.

Tracked at scala/scala3#18484

[MateuszKubuszok]

I need this refactors and fixes in other branches, so I cannot wait indefinite time for being able to have this one test being handled properly on Scala 3.