This commit makes several modifications to the modules command. It
downcases the field names in the resulting JSON string, adds a format
version to the result. The result of this command will now only contain
entries which have been declared in configuration, as opposed to the
internal manifest which shows all installed modules for a working dir.
The modules command provides a holistic view of all installed modules
for a given root module. It also includes information on whether a
installed module is currently referenced by configuration. As of now,
this command only supports a machine-readable view format.
