Create composite client bundles directly under target (rebased onto develop)


This is the same as gh-2737 but rebased onto develop.

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The target/pkg subdirectory is most certainly historical and certainly not
a requirement anymore. This commit modifies the release-clients build target
to move all the OMERO.clients zip directly under target instead.

To test this PR, make sure the build still passes and check the artifacts of the various CI builds. The OMERO.clients bundles should now be located at the same level as the OMERO.insight bundles and should still be fully functional.