[core] Make ClasspathClassLoader::getResource child first

[oowekyala]

Describe the PR

This makes it so that one can use the auxclasspath to include a standard lib for a different java version than the one on PMD's own classpath.

Related issues

• Related to [java] UnnecessaryCast reports false-positive for ByteBuffer methods when targeting Java 8 #4620

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)

[adangel]

Thanks!

I'll look at the failing Windows unit test (ClassLoadingChildFirstTest.testClassLoading:41 � InvalidPath Illegal char <:> at index 2: /D:/a/pmd/pmd/pmd-java/target/test-classes/net/sourceforge/pmd/lang/java/symbols/custom_java_lang.jar) and merge it afterwards.

[pmd-test]

	1 Message
📖	Compared to master: This changeset changes 0 violations, introduces 0 new violations, 0 new errors and 0 new configuration errors, removes 2 violations, 0 errors and 0 configuration errors. Full report

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