By adding this method you now only have to pass a `*disco.Disco` object around in order to do discovery and use any configured credentials for the discovered hosts.
Of course you can also still pass around both a `*disco.Disco` and a `auth.CredentialsSource` object if there is a need or a reason for that!
The default network-based discovery is not desirable for all situations,
so this mechanism allows callers to provide a services map for a given
hostname that was obtained some other way (caller-defined) which will then
cause network-based discovery to be skipped and the given map to be
returned verbatim.
Although service discovery metadata is usually not sensitive, a service
host may wish to produce different results depending on the requesting
user, such as if users are migrating between two different implementations
that are both running concurrently for some period.
This package implements our Terraform-native Service discovery protocol,
which allows us to find the base URL for a particular service given a
hostname that was already validated and normalized by the svchost package.
Sander van Harmelen