AutoMLSearch: rename "data_split" to "data_splitter"

[dsherry]

Came up in #1568 , seemed like a nice clarification to make.

How I made this PR (on mac):

git grep -il "data_split" . | xargs sed -i '' -e "s/data_split/data_splitter/g"
git grep -il "data_splitterter" . | xargs sed -i '' -e "s/data_splitterter/data_splitter/g"

[codecov]

Codecov Report

Merging #1569 (72575f2) into main (edb6ed6) will not change coverage.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##             main    #1569   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         240      240           
  Lines       18092    18092           
=======================================
  Hits        18084    18084           
  Misses          8        8           

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.7% <100.0%> (ø)
evalml/automl/utils.py	100.0% <100.0%> (ø)
evalml/tests/automl_tests/test_automl.py	100.0% <100.0%> (ø)
.../automl_tests/test_automl_search_classification.py	100.0% <100.0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update edb6ed6...72575f2. Read the comment docs.

[jeremyliweishih]

cool!