* Allow branching out of the "generic build" unification set
For cases like the one in https://github.com/spack/spack/pull/39661
we need to relax rules on unification sets.
The issue is that, right now, nodes in the "generic build" unification
set are unified together with their build dependencies. This was done
out of caution to avoid the risk of circular dependencies, which would
ultimately cause a very slow solve.
For build-tools like Cython, however, the build dependencies is masked
by a long chain of "build, run" dependencies that belong in the
"generic build" unification space.
To allow splitting on cases like this, we relax the rule disallowing
branching out of the "generic build" unification set.
* Fix issue with pure build virtual dependencies
Pure build virtual dependencies were not accounted properly in the
list of possible virtuals. This caused some facts connecting virtuals
to the corresponding providers to not be emitted, and in the end
lead to unsat problems.
* Fixed a few issues in packages
py-gevent: restore dependency on py-cython@3
jsoncpp: fix typo in build dependency
ecp-data-vis-sdk: update spack.yaml and cmake recipe
py-statsmodels: add v0.13.5
* Make dependency on "blt" of type "build"
e20c05fcdf