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!
Unlike the old installer in config/module, this uses new-style
installation directories that include the static module path so that paths
we show in diagnostics will be more meaningful to the user.
As before, we retrieve the entire "package" associated with the given
source string, rather than any given subdirectory directly, because the
retrieved module may contain ../ references into parent directories which
must be resolvable after extraction.
The level of abstraction that needs the "svchost" is the Module, not the
FriendlyHost. Us the new method in the module package for registry
interaction.
The detection of registry modules will have to happen in mutliple
phases. The go-getter interface requires that the detector return the
final URL, while we won't know that until we verify which version we
need. This leaves the regisry sources broken, to be re-integrated in a
following commit.
Removed some of the test cases that we don't allow in the svchost
package. Will check back if those are needed in the registry and work
around them as necessary.
