Instead of bind-mounting each individual input into the container,
create a temporary directory that is used by all inputs and bind-
mount this to the well known location ("/run/osbuild/inputs"). The
temporary directory is then passed to the input so that it can
make the requested resources available relative to that directory.
This is enforced by the common input handling code.
Additionally, pass the well known input path via a new "paths" key
to the arguments dictionary passed to the stage.
Use `ostree-ouput` instead of `files-output` as prefix for the
temporary directory prefix.
Also fix the description of the "origin" to reflect that
pipeline and source origins are supported.
Additionally remove some dead code.
Add a new input that can handle ostree commits. For source origins,
it will pull any number of commits specified via `references`. For
pipeline origins it will use the compose.json to obtain the commit
id and pull that. Either way the data passed onto the stage is a
dictionary of commit ids and commit metadata, such as the `ref`
that is an ostree ref that is optionally created in case the `ref`
option for that reference was specified.
