Clean up and refactor InvalidTargetDataCheck

[angela97lin]

Closes #1741

This PR adds a more in-depth docstring detailing what InvalidTargetDataCheck checks, and breaks up each of the different checks into their own helper methods.

[codecov]

Codecov Report

Merging #3122 (baecc19) into main (b9d4cb6) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3122     +/-   ##
=======================================
+ Coverage   99.8%   99.8%   +0.1%     
=======================================
  Files        315     315             
  Lines      30664   30684     +20     
=======================================
+ Hits       30573   30593     +20     
  Misses        91      91             

Impacted Files	Coverage Δ
...ata_checks_tests/test_invalid_target_data_check.py	100.0% <ø> (ø)
evalml/data_checks/__init__.py	100.0% <100.0%> (ø)
evalml/data_checks/default_data_checks.py	100.0% <100.0%> (ø)
evalml/data_checks/invalid_target_data_check.py	100.0% <100.0%> (ø)
..._tests/test_data_checks_and_actions_integration.py	100.0% <100.0%> (ø)

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[freddyaboulton]

Thanks @angela97lin ! Looks good to me!

[chukarsten]

Looks good to me. Left some minor comments but feel free to ignore.

[eccabay]

I'm not sure, but would this count as a breaking change?

[angela97lin]

Oo good call! I'd say it's safer to add it as a breaking change since we're changing the module name. Adding to release notes, thanks!