RFR: 7903720: Change layout of jextract image

[Jorn Vernee]

See JBS issue for problem description.

This patch changes the layout of the jextract image we produce. The new image has the 2 launcher scripts in the root of the image, with a nested `runtime` folder containing the jlink runtime image.

I've added both launcher scripts to the repo under the `src/main` directory, and they are copied to the image during the build. The script is somewhat simplified by removing a blank variable that was not being used. I also took the opportunity to address an issue with the windows launcher script turning off `echo`. Instead it now avoids output using the `@` prefix for each command in the script.

I've tested that the scripts work (using `jextract --help`) on both Linux and Windows.

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Commit messages:
 - remove spurious imports
 - update verify task
 - Improve image layout and launcher scripts

Changes: https://git.openjdk.org/jextract/pull/241/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=241&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903720
  Stats: 18 lines in 4 files changed: 12 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jextract/pull/241.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/241/head:pull/241

PR: https://git.openjdk.org/jextract/pull/241