RFR: 8365555: Cleanup redundancies in jpackage implementation

[Alexey Semenyuk]

On Thu, 14 Aug 2025 13:35:50 GMT, Alexey Semenyuk <asemenyuk at openjdk.org> wrote:

> - Remove AppImageDesc class. It was supposed to bind an AppImageLayout instance with a root directory, but since AppImageLayout has `rootDirectory()` method it is redundant.
>  - Remove redundant `Package.packageLayout()` and `Package.asPackageApplicationLayout()` methods from the model, 
>  - Add tests for PackagingPipeline class.
>  - Move JUnitAdapter.java from "/test/jdk/tools/jpackage/helpers-test" to "/test/jdk/tools/jpackage/junit/tools" directory, support running platform-specific JUnit tests.
>  - Enhance AppImageLayout class, add unit tests for derived MacApplicationLayout and LinuxApplicationLayout classes.
>  - Add tests for BuildEnv class.

I'd appreciate a better alternative for the title.

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PR Comment: https://git.openjdk.org/jdk/pull/26778#issuecomment-3189854226