RFR: 8315130: java.lang.IllegalAccessError when processing classlist to create CDS archive

[Timofei Pushkin]

If a base class is package-private then its subclasses should have the same package name and defining class loader, otherwise `IllegalAccessError` is thrown when linking a subclass. Currently when dumping a static archive separate `URLClassLoader`s are used for each unregistered classes' source. Thus if two unregistered classes, a package-private base class and a sub class, from the same package reside in different sources `IllegalAccessError` will be thrown when linking the sub class. This can be unexpected because the app could have used a single class loader for both classes and thus not have seen the error — see `DifferentSourcesApp.java` from this patch for an example of such app.

This patch fixes the issue by using a single class loader for all unregistered classes. CDS does not allow classes with the same name making such solution possible.

Implementation note: `URLClassLoader` does not allow selecting a specific URL to load a specific class — I used reflection to override a private part of `URLClassLoader` responsible for URL selection while being able to use the rest of its implementation.

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Commit messages:
 - Add regression test
 - Use a single UnregisteredClassLoader

Changes: https://git.openjdk.org/jdk/pull/24223/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315130
  Stats: 448 lines in 11 files changed: 276 ins; 98 del; 74 mod
  Patch: https://git.openjdk.org/jdk/pull/24223.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24223/head:pull/24223

PR: https://git.openjdk.org/jdk/pull/24223