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!
Originally the hope was to use the afero filesystem abstraction for all
loader operations, but since we install modules using go-getter we cannot
(without a lot of refactoring) support vfs for installation.
The vfs use-case is for reading configuration from plan zip files anyway,
and so we have no real reason to support installation into a vfs. For now
at least we will just add the possibility that a loader might not be
install-capable. At the moment we have no non-install-capable loaders, but
we'll add one later once we get to loading configuration from plan files.
Previously the behavior for loading and installing modules was included in
the same package as the representation of the module tree (in the
config/module package).
In our new world, the model of a module tree (now called a "Config") is
included in "configs" along with the Module and File structs. This new
package replaces the loading and installation functionality previously
in config/module with new equivalents that work with the model objects
in "configs".
As of this commit, only the loading functionality is implemented. The
installation functionality will follow in subsequent commits.
Radek Simko