RFR: 8343416: CDS dump fails when unregistered class can also be loaded from system modules

[Calvin Cheung]

For classes to be loaded by a custom class loader during dump time, we currently use the `URLClassLoader.loadClass` to load the class which will be delegated to the boot class loader if the class is from a system module jar (e.g. jrt-fs.jar). To avoid the delegation and load the class by a custom class loader, this fix uses the `URLClassLoader.findClass` instead.

Passed tiers 1 - 3 testing.

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Commit messages:
 - 8343416: CDS dump fails when unregistered class can also be loaded from system modules

Changes: https://git.openjdk.org/jdk/pull/22049/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22049&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343416
  Stats: 57 lines in 2 files changed: 54 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22049.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22049/head:pull/22049

PR: https://git.openjdk.org/jdk/pull/22049