[Ingest Manager] Rename and restructure data source saved object

[jen-huang]

PR: #70259

Summary

As part elastic/beats#19082, datasources have been removed from package registry and agent config yaml. There are already some Kibana changes to support this removal: #69226, #69864

The existing Kibana changes so far have not significantly modified the datasource saved object, nor were any changes done to remove the word datasources from the code base (variable names, file names, routing paths, etc).

As we will not be supporting an upgrade path for Ingest Manager from alpha to beta release, we should handle the removal and restructure of datasource saved objects ahead of beta release, lest we risk breaking changes after beta 😬

Tasks

Note that this task list does not include anything that significantly impacts the UI. Removing the term data sources from the UI will be handled in a different issue following the implementation of this one.

• Rename ingest-datasources saved object type to ingest-package-configs
  • Remove existing datasource SO migrations as we are not supporting a migration path from alpha to beta
• Assess existing SO mappings and simplify as much as possible, while still retaining the necessary structure to convert (package definition + user-defined var values) -> agent config yaml
  • It is likely that we can remove one level from current mappings As it turns out, we cannot :)
• Update naming everywhere
  • Variable names
  • File names
  • Routing paths
  • ...and probably more :)
• Adjust Endpoint usages of anything datasource-related

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[hbharding]

I created this issue for tracking names we want to change in the UI #70024