RFR: 8336470: Source launcher should work with service loader SPI in unnamed module

[Christian Stein]

Please review this change adding a missing resource file test to the `MemoryClassLoader::findResource` override in order to resolve all files beneath the source root path in source launch mode - analog to what was already implemented for `MemoryClassLoader::getResource`.

With this change applied, files in `META-INF/services/` are resolved; allowing services providers being supplied in source form.

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Commit messages:
 - 8336470: Source launcher should work with service loader SPI in unnamed module

Changes: https://git.openjdk.org/jdk/pull/20193/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20193&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336470
  Stats: 39 lines in 3 files changed: 23 ins; 4 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20193.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20193/head:pull/20193

PR: https://git.openjdk.org/jdk/pull/20193