[v2] Move providers/ to internal/providers

[andrewkroh]

The github.com/elastic/go-sysinfo/providers/packages should be treated as internal, and I'd like to make this official by moving them to internal/providers in the next "v2" major release (timeline TBD). This way, the only API surface is the top-level API. This will give us more flexibility in refactoring and force users to go through the front door as it was designed.