Handle new archive structure in Upgrade handler

[michalpristas]

PR #714 introduced a new archive structure which prevents performing upgrades with the current upgrade code

Upgrade handler needs to be able to detect type of the target where /Library/Elastic/Agent/elastic-agent links to

It's either a binary and linking is fine as it is
or it's *.app and we need to link binary buried inside this directory at elastic-agent.app/Contents/MacOS/{{.BeatName}}{{.BinaryExt}}

This change needs to be backported to 8.5 so it can handle updated packages from 8.6

[michalpristas]

closing in favor of #1389