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Serialisation was simple when we only had 1 artifact per source.
However, to allow smaller systems, we need artifact splitting to produce
multiple artifacts per chunk source.
So now the new serialisation format has a separate list of artifacts
and sources, rather than the Source being generated from the artifact's
serialisation.
Python's id() function is used to encode the references between the
various Sources and Artifacts, these are replaced with a reference to
the new object after deserialisation.
Previously the cache-key was used, but this is no longer sufficient to
uniquely identify an Artifact.
The resultant build graph after deserialisation is a little different
to what went in: Strata end up with a different Source per Artifact,
so it _is_ a 1 to 1 mapping, as opposed to Chunks, where it's many to 1.
We serialise strata and chunks differently because stratum artifacts
from the same source can have different dependencies, for example
core-devel can have different dependencies to core-runtime.
Without intervention we would serialise core-devel and core-devel's
dependencies without including core-runtime's dependencies.
To solve this we've decided to encode stratum artifacts completely
indepedently: each stratum artifact has its own source. This is safe
because stratum artifacts can be constructed independently,
as opposed to Chunks where all the Artifacts for a Source
are produced together.
This is a little hacky in its current form, but it simplifies matters
later in distbuild with regards to how it handles expressing that
every Artifact that shares a Source is built together.
Arguably, this should be the output of producing the build graph
anyway, since it more helpfully represents which Artifacts are built
together than checking the morphology kind all the time, but more
assumptions need checking in morph before it's safe to make this
change across the whole of the morph codebase.
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