[Fleet] Requiring gloablly unique package-policy names is unexpected

[simitt]

Fleet does not allow to set up multiple package policies with the same name. It is unexpected that package policy names need to be globally unique. Given that package policies are always added to an agent policy, it seems more plausible that the uniqueness check on the name is per agent policy instead of a global one.

Problems might occur on cloud when users have already added an APM or a Fleet Server to any agent policy with the same name. The package policy names used in cloud are very generic (Fleet Server, Elastic APM), so it wouldn't be surprising if the names were already taken. On the other hand, we don't want to introduce any random number etc. to the name for cloud policies. Changing the names at this point would also bring issues with upgrades of existing policies.

Another scenario I can think of is if users create agent policies per environment, let's say a staging and a production agent policy, both containing a fleet-server. They would need to either add some randomness or repeat the environment name in the package policy, for example name it Fleet Server (production) and Fleet Server (staging), adding some redundancy.

The name is intended to help the user quickly identify which package policy is doing what. If a globally wide distinction is necessary, it can always be combined with the agent policy name.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[simitt]

cc @caitlinbetz for visibility, as the original request for global uniqueness came from #72948