Move remaining code out of legacy core/logical_plan module

[andygrove]

Which issue does this PR close?

Part of #2683

Rationale for this change

The core/logical_plan module is now redundant and mostly consists of re-exports. There were two items remaining in this module that are not referenced from logical plans and should not be in this module. These items are DefaultTableSource and FunctionRegistry.

What changes are included in this PR?

• Move DefaultTableSource to datasource module
• Move FunctionRegistry to core/execution module

Are there any user-facing changes?

No. I added public re-exports.

[codecov-commenter]

Codecov Report

Merging #2701 (7d350ac) into master (9306534) will increase coverage by 0.00%.
The diff coverage is 84.61%.

@@           Coverage Diff           @@
##           master    #2701   +/-   ##
=======================================
  Coverage   84.66%   84.66%           
=======================================
  Files         270      271    +1     
  Lines       46919    46919           
=======================================
+ Hits        39724    39725    +1     
+ Misses       7195     7194    -1     

Impacted Files	Coverage Δ
datafusion/core/src/datasource/mod.rs	93.93% <ø> (ø)
datafusion/core/src/logical_plan/plan.rs	94.82% <ø> (+1.02%)	⬆️
datafusion/core/src/physical_plan/planner.rs	77.51% <ø> (ø)
...fusion/core/src/datasource/default_table_source.rs	84.61% <84.61%> (ø)
datafusion/optimizer/src/utils.rs	31.72% <0.00%> (-1.08%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	68.73% <0.00%> (+0.77%)	⬆️

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[alamb]

Makes sense to me -- thank you @andygrove