Spack currently tries to unify everything in the DAG, but this is too strict for build dependencies, where it is fine to build a dependency with a tool that conflicts with a version fo that tool for a dependent's build.
To enable a workaround for conflicts among build dependencies, so that users can install in multiple steps to avoid these conflicts, make the following changes:
* Dont apply package dependency constraints for build deps of installed packages
* Avoid applying constraints for installed packages vs. concrete packages
* Mark all dependencies of installed packages as visited in normalization method
* don't remove dependency links for concrete specs in flat_dependencies
Also add tests:
* Update test to ensure that link dependencies of installed packages have constraints applied
* Add test to check for proper handling of transitive dependencies (which is currently not the case)
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