[Fleet] Adding an agent policy with a unique name triggers an error

[criamico]

Kibana version:
8.0.0

Describe the bug:
Adding an agent policy with "Collect system logs and metric" checkbox enabled triggers the error There is already a package with the same name. This doesn't happen when adding the first agent policy (like the first time the system gets setup), but happens subsequently, likely because the checkbox automatically adds a package policy for system.

Hence the logic adding thesystem package when system Monitoring is enabled needs to be updated to respect the new requirements (for instance by incrementing the number in the name, like here).

Steps to reproduce:

1. Navigate to "add agent policy" fly out
2. If it's the first time you set up the system, add an agent policy with name test , otherwise go to step 3
3. Now add another agent policy with name test2, or any other name as long as is unique with "Collect system logs and metric" checked
4. The error "There is already a package with the same name" will pop up, but the agent policy is added regardless

Expected behavior:
When the agent policy name is unique no error should appear.

Screenshots:

Notes
Bug caused by the merging of #115212, which modified the functionality around the naming of package policies (they should be unique across all agent policies).

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)