Minor: rename parquet.rs to parquet/mod.rs #8301
                
     Merged
            
            
          
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Which issue does this PR close?
N/A
Rationale for this change
While working with the file format code, I noticed that the
parquet.rsmodule definition was in a different directory than its submodules which was confusing and not consistent with the rest of DataFusion's codebasehttps://github.com/apache/arrow-datafusion/tree/main/datafusion/core/src/datasource/physical_plan
What changes are included in this PR?
Move parquet to be with its submodules
Are these changes tested?
Are there any user-facing changes?