We cannot import the packages from all of these three packages sets into
the global namespace, because they are indistinguishable. For example:
$ nix-env -qaP \* | grep pylint
pypyPackages.pylint pylint-0.26.0
python33Packages.pylint pylint-0.26.0
python27Packages.pylint pylint-0.26.0
When someone tries to install pylint by running "nix-env -i pylint",
then it's impossible to tell which one of these three versions was
chosen.
I can think of two ways to remedy this problem with recurseIntoAttrs:
1) Bake the name of the Python interpreter into every package's name,
i.e. offer "python27-pylint", "python33-pylint", and so on.
2) Ensure that all non-default package sets mark all their packages
'lowPrio' to unsure that the choice during installation is
deterministic.
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