Validate source references while loading manifests so that a bad
reference would result in a meaningful error message instead of a
hard-to-understand Python exception.
The `org.osbuild.files` source provides files, but might in the
future not be the only one that does. Therefore rename it to
match the internal tool that is being used to fetch the files.
This is done for most other osbuild modules that target tools.
The format v1 loader is adapted to make this change transparent
for users of the v1 format, so we are backwards compatible.
Change the MPP depsolve preprocessor so that for format v2 based
manifest `org.osbuild.curl` source is used. Also rename the
corresponding source test. Adapt the format v2 mod test to use
the curl source.
Add a basic check to verify that loading and then describing the
pipeline results in the same description that was put in. This
test is esp. valuable because it checks the runner mapping and
name, id mappings.
Add a new test to check that validation works for the basic test
pipeline. This needs to be extended in the future to check that
invalid data is being caught properly, but it is a start.
