fix(eslint-plugin): skip seenTypes for unions in isTypeReadonly

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: fixes [prefer-readonly-parameter-types] Maximum call stack size exceeded #3396
• Steps in CONTRIBUTING.md were taken

Overview

If a type within a union type was already added to seenTypes, we don't need to recurse into it.

[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

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The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[nx-cloud]

Nx Cloud Report

CI ran the following commands for commit 4693b87. Click to see the status, the terminal output, and the build insights.

📂 See all runs for this branch

Status	Command
	Node 12.x - nx test @typescript-eslint/eslint-plugin-internal
	Node 12.x - nx test @typescript-eslint/eslint-plugin-tslint
	Node 12.x - nx test @typescript-eslint/eslint-plugin
	Node 12.x - nx test @typescript-eslint/eslint-plugin-tslint
	Node 12.x - nx test @typescript-eslint/eslint-plugin
	Node 16.x - nx test @typescript-eslint/eslint-plugin-tslint
	Node 16.x - nx test @typescript-eslint/eslint-plugin
	Node 12.x - nx run-many --target=typecheck --all --parallel
	Node 12.x - nx test @typescript-eslint/parser
	Node 12.x - nx test @typescript-eslint/experimental-utils
	Node 12.x - nx test @typescript-eslint/scope-manager
	Node 12.x - nx test @typescript-eslint/visitor-keys
	Node 12.x - nx test @typescript-eslint/typescript-estree

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[codecov]

Codecov Report

Merging #4043 (4693b87) into master (f418f48) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4043   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files         152      152           
  Lines        8014     8014           
  Branches     2568     2569    +1     
=======================================
  Hits         7480     7480           
  Misses        180      180           
  Partials      354      354           

Flag	Coverage Δ
unittest	93.33% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/eslint-plugin/src/util/isTypeReadonly.ts	94.36% <100.00%> (ø)

[bradzacher]

noiiice