Make the sources options a static property of the pipeline, in
particular of each stage, rather than being passed in on `run()`.
This more closely matches the intended semantics of sources and
pipeline having similar lifetimes and being fairly coupled together.
The difference between the pipeline and the sources is that the
sources do not contribute to identifying the pipeline (they are not
part of the hash for the pipeline id), and they could be swapped
out without changing the output image (as long as they are valid).
However, a pipeline without A sources object would not be useful,
and typically the pipeline and the sources are generated, passed
around and used together.
This is different from the build environment and the secrets object,
which both are specific to either the host or the caller, unlike
the pipeline which should be universal.
This changes the `load()` function to take a `manifest`, which is
a map containing both the pipeline and the sources.
Note that the semantics of the build-env parameter remains unchanged:
It shares the sources with the rest of the pipeline. We may want to
reconsider this in future commits, as the build-env is specific to
the host, whereas the regular pipeline is not.
Signed-off-by: Tom Gundersen <teg@jklm.no>
481213a8dd