Normally when using external packages in concretization, Spack ignores
all dependencies of the external. #33777 updated this logic to attach
a Python Spec to external Python extensions (most py-* packages), but
as implemented there were a couple issues:
* this did not account for concretization groups and could generate
multiple different python specs for a single DAG
* in some cases this created a fake Python spec with insufficient
details to be usable (concretization/installation of the
extension would fail)
This PR addresses both of these issues:
* For environment specs that are concretized together, external python
extensions in those specs will all be assigned the same Python spec
* If Spack needs to "invent" a Python spec, then it will have all the
needed details (e.g. compiler/architecture)
08101639cd