resolver: stabilize output for tests (& users)

[xavfernandez]

Otherwise the test_install_distribution_union_with_versions test can end
up with either:

Cannot install localextras[bar] 0.0.2 and localextras[baz] 0.0.1 because these package versions have conflicting dependencies.

or

Cannot install localextras[baz] 0.0.2 and localextras[bar] 0.0.1 because these package versions have conflicting dependencies.

[xavfernandez]

An alternative could be to replace

pip/tests/functional/test_install_reqs.py

Line 585 in cdc5422

assert "Cannot install localextras[bar]" in result.stderr

by assert "Cannot install localextras" in result.stderr

But explicitly stabilizing the output might be worthwhile on its own (except if the current order is meaningful, which does not seem the case in Python 3.5).

[pradyunsg]

Did I not change this already in #9019? Huh.