Define Base.deepcopy_internal() instead of Base.deepcopy()

[JamesWrigley]

As recommended by the deepcopy() docs. Fixes these invalidations seen when loading CurveFit.jl on 1.13:

 inserting deepcopy(f::RuntimeGeneratedFunctions.RuntimeGeneratedFunction) @ RuntimeGeneratedFunctions ~/.julia/packages/RuntimeGeneratedFunctions/odmYb/src/RuntimeGeneratedFunctions.jl:582 invalidated:                                     
   backedges: 1: superseding deepcopy(x) @ Base deepcopy.jl:32 with MethodInstance for deepcopy(::Any) (43 children)                                                                                                                           
   1 mt_cache                                                                                                                                                                                                                                  

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API.