[Fleet] Input Packages Phase 2 - Ensure upgrade from package with type: integration to type: input is supported

[kpollich]

Child of #133296

Phase 2 Goals

• A user can install the latest version of a package which includes a change from type: integration to type: input
• A user can upgrade their existing policies for such a package and maintain their variable definitions, ingest customizations, etc

Implementation Checklist

• Updating the version of a package from the integration details page when the current version is type: integration and the new version is type: input works as expected
• Upgrading a package policy from a prior type: integration version of a package to a newer type: input package works as expected including
  • Conflict detection (e.g. new required vars with no defaults)
  • Saving the policy persists the version change as expected

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Reopening since we've had some breakages related to our changes here in the policy editor: #141705