[Fleet] Reduce fields returned by get package info API

[hop-dev]

Now that we have the ?full parameter for the get package info endpoint, we get the package info for the policy editor from the package archive, this is the "full" package info.

Currently the full package info only has a couple of extra fields, which makes it hard to distinguish, both in the code and from an API point of view. I am proposing that we strip down the get package info API to only provide the information needed to display the package summary page.

For example:

• remove data streams
• remove policy templates

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)