Desktop: Fixes #9781: Extract default plugins to directories matching their IDs

[personalizedrefrigerator]

Summary

Rather than storing default plugin JPL files in

    build/defaultPlugins/[[ plugin ID here ]]/plugin.jpl

this pull request stores default plugin JPL files in

    build/defaultPlugins/[[ plugin ID here ]].jpl

Because PluginService uses the name of the .jpl file to determine where to extract it, this change fixes #9781.

Testing

1. Clear Joplin's cache directory (~/.config/joplin-desktop/cache).
2. Start Joplin
3. Inspect Joplin's cache directory
   • Verify that there exists a folder named io.github.jackgruber.backup/ and not a folder named plugin/

This has been tested successfully on Ubuntu 23.10.

Note: That the Simple Backup plugin starts successfully is tested by this set of automated tests.

[personalizedrefrigerator]

This pull request adjusts the default plugins tests so that they continue to pass.

Previously, the ${supportDir}/pluginRepo/plugins directory was used (which is also used for other tests). However, it has the following structure:

packages/app-cli/tests/support
| pluginRepo/
|   | plugins/
|   |   | joplin.plugin.ambrt.backlinksToNote/
|   |   |   | plugin.jpl
|   |   |   | manifest.json
|   |   | org.joplinapp.plugins.ToggleSidebars/
|   |   |   | plugin.jpl
|   |   |   | manifest.json
|   | ...

Because this pull request changes the format of the default plugins directory, a new ${supportDir}/testDefaultPlugins directory is created with the expected structure:

packages/app-cli/tests/support
| testDefaultPlugins/
|   | joplin.plugin.ambrt.backlinksToNote.jpl
|   | org.joplinapp.plugins.ToggleSidebars.jpl