Fix --package-registry-apm-path flag

[axw]

In elastic/apm-server#5414 the apm-server/apmpackage/apm package version directory was removed from VCS. This means the package cannot be directly loaded into package-registry; it needs to be copied into a version directory first, e.g. like in https://github.com/elastic/apm-server/blob/master/testing/docker/package-registry/entrypoint.sh