get_remote_repositories()'s API was using plan IDs as a way of

identifying users vs. organizations, which led to some redundancy.

For example, "private" and "public" for GitHub meant the same thing

(users). GitHub in particular was assuming certain filter types for

repositories, which would be handy to specify from a caller's point of

view.
The function has now been changed to remove the plan parameter and to

instead have a dedicated "owner_type" parameter that specifies what

"owner" refers to. The value is dependent on the hosting service, and

HostingService subclasses are expected to be smart about the defaults.
It also now takes a "filter_type" parameter that allows for filtering

repositories. Possible values are dependent on the hosting service as

well. For instance, GitHub's allowed values consist of what their API

natively allows for their "type=" parameter in repository queries.

Unit tests pass.
Tested with my upcoming web API change, testing user and org lookups

and various filters.