[EPM] Update paths to new structure from package registry

[neptunian]

This PR, changes the path structure from {package}-{version} to {package}/{version}. There are some places in EPM that will need to be updated. Here are some but there are probably more:

https://github.com/elastic/kibana/blob/master/x-pack/plugins/ingest_manager/server/services/epm/registry/index.ts#L102
https://github.com/elastic/kibana/blob/master/x-pack/plugins/ingest_manager/server/services/epm/packages/assets.ts#L38

The task is to get EPM working locally with the local PR of the registry and publish the changes together.

[elasticmachine]

Pinging @elastic/ingest-management (Feature:EPM)

[jen-huang]

FYI, I recall concatenating package name+version for some EPM API calls in agent config UI. Not sure if we are also planning on changing EPM APIs to /name/version as part of this issue too?

[neptunian]

@jen-huang Based on some conversation, we have decided not to, so the EPM api will not change.