Fixed RAT check.

[jorgecarleitao]

Closes #481

[jorgecarleitao]

Notice that I made it such that it uses the apache-rat instead of archery; this way we do not depend on archery for this.

I am not sure whether this is sufficient, though; I assume that archery just calls the apache-rat.

Regardless, imo this is better than what we have, where we currently do not even check for the headers of .rs files.

[jorgecarleitao]

Build log on my repo after removing the license from arrow/src/compute/kernels/aggregate.rs, on top of this PR: https://github.com/jorgecarleitao/arrow-rs/runs/2879443315?check_suite_focus=true

Run ./dev/release/run-rat.sh .
NOT APPROVED: arrow/src/compute/kernels/aggregate.rs (./arrow/src/compute/kernels/aggregate.rs): false
1 unapproved licences. Check rat report: rat.txt
Error: Process completed with exit code 1.

[codecov-commenter]

Codecov Report

Merging #482 (b9ca4f5) into master (9f56afb) will decrease coverage by 0.00%.
The diff coverage is 98.70%.

❗ Current head b9ca4f5 differs from pull request most recent head 7a86ee0. Consider uploading reports for the commit 7a86ee0 to get more accurate results

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   82.65%   82.65%   -0.01%     
==========================================
  Files         165      165              
  Lines       45556    45524      -32     
==========================================
- Hits        37655    37628      -27     
+ Misses       7901     7896       -5     

Impacted Files	Coverage Δ
parquet/src/encodings/decoding.rs	92.47% <ø> (ø)
parquet/src/arrow/array_reader.rs	77.45% <57.14%> (+0.13%)	⬆️
arrow/src/json/reader.rs	84.02% <97.56%> (+0.11%)	⬆️
arrow/src/array/array_binary.rs	90.13% <100.00%> (ø)
arrow/src/array/array_boolean.rs	90.90% <100.00%> (ø)
arrow/src/array/array_dictionary.rs	84.56% <100.00%> (ø)
arrow/src/array/array_primitive.rs	94.70% <100.00%> (ø)
arrow/src/array/array_struct.rs	87.84% <100.00%> (-0.59%)	⬇️
arrow/src/array/array_union.rs	86.93% <100.00%> (ø)
arrow/src/array/builder.rs	85.78% <100.00%> (-0.13%)	⬇️
... and 22 more

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[alamb]

Looks great -- thank you @jorgecarleitao