[multistage][refactor] clean up planner

[walterddr]

• plan fragment metadata is not needed, we use dispatchablePlanFragment & dispatchablePlanMetadata
• move Dispatchable* class into physical package
• move Explain in to explain package

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c57117a) 0.00% compared to head (7a8a2a9) 61.60%.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #12070       +/-   ##
=============================================
+ Coverage      0.00%   61.60%   +61.59%     
- Complexity        6     1153     +1147     
=============================================
  Files          2314     2389       +75     
  Lines        126078   129819     +3741     
  Branches      19524    20082      +558     
=============================================
+ Hits              6    79970    +79964     
+ Misses       126072    44025    -82047     
- Partials          0     5824     +5824     

Flag	Coverage Δ
custom-integration1	<0.01% <0.00%> (ø)
integration	<0.01% <0.00%> (ø)
integration1	<0.01% <0.00%> (?)
integration2	0.00% <0.00%> (?)
java-11	61.57% <100.00%> (+61.56%)	⬆️
java-21	34.88% <100.00%> (+34.88%)	⬆️
skip-bytebuffers-false	61.59% <100.00%> (?)
skip-bytebuffers-true	34.87% <100.00%> (?)
temurin	61.60% <100.00%> (+61.59%)	⬆️
unittests	61.59% <100.00%> (?)
unittests1	46.88% <100.00%> (?)
unittests2	27.60% <0.00%> (?)

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