When using an external package with the old concretizer, all
dependencies of that external package were severed. This was not
performed bidirectionally though, so for an external package W with
a dependency on Z, if some other package Y depended on Z, Z could
still pull properties (e.g. compiler) from W since it was not
severed as a parent dependency.
This performs the severing bidirectionally, and adds tests to
confirm expected behavior when using config from DAG-adjacent
packages during concretization.
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