RFR: 8370123: Minor jpackage refactoring [v2]

[Alexey Semenyuk]

> Minor changes, no functional impact.
> 
> - ApplicationBuilder: add externalApplication() and mainLauncherClassName() methods
> - `AppImageSigner.normalizeAppImageLayout()` -> `AppImageSigner.copyWithUnresolvedAppImageLayout()`
> - Add `JLinkRuntimeBuilder.ensureBaseModuleInModulePath()`; move code away from StandardBundlerParam; remove redundant "warning.no.jdk.modules.found" string ID
> - Add `OverridableResource.probe()` method
> - Remove dead and "odd" code from IOUtils

Alexey Semenyuk has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains nine commits:

 - Merge branch 'master' into JDK-8370123
 - JLinkRuntimeBuilder: remove trailing whitespace
 - JLinkRuntimeBuilder: add a comment
 - IOUtils: remove dead code; remove getFileName(), getParent() functions.
 - ApplicationLaunchers: minor
 - OverridableResource: add probe() method
 - Add JLinkRuntimeBuilder.ensureBaseModuleInModulePath()
 - AppImageSigner: better function name
 - ApplicationBuilder: add externalApplication() and mainLauncherClassName() methods

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Changes: https://git.openjdk.org/jdk/pull/27873/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27873&range=01
  Stats: 241 lines in 13 files changed: 83 ins; 141 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/27873.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27873/head:pull/27873

PR: https://git.openjdk.org/jdk/pull/27873