extras not handled correctly when a distribution occurs twice in the dependency graph

[rbtcollins]

For both the special case of on-the-command-line:

pip install foo foo[bar]

which will error with 'duplicate requirement'

and the general case of two references in the dependency graph:

pip install foo

where foo depends on bar and quux, and quux depends on bar[extra]

which will fail to install whatever is listed in bar[extra].

For all these cases we should:

• union the extras in the install requirement object
• recurse into the dependencies specified by the extra (which probably means recursing into everything due to pkg_resources api, but thats an implementation detail).

[dims]

Specific scenario in openstack, we have oslo.db has an extras named 'fixture' which has a couple of libraries used only in tests. we'd like to add oslo.db[fixtures] to either test-requirements or tox.ini when oslo.db is already present in requirements.txt .

Currently pip fails with:
Double requirement given: oslo.db[fixtures]>=2.4.1 (from -r /Users/dims/openstack/openstack/nova/test-requirements.txt (line 17)) (already in oslo.db>=2.4.1 (from -r /Users/dims/openstack/openstack/nova/requirements.txt (line 40)), name='oslo.db')

http://paste.openstack.org/show/476434/

[piotr-dobrogost]

What is called general case here matches what I rised in issue #3147 and according to @xavfernandez pull #2716 by @rbtcollins :) is supposed to fix this.

[rbtcollins]

#3105 is at least partially fixing this.

My resolver branch can fix 'correctly' this, but we don't need to wait for it to address the basic defect (since we can union all extras together easily enough).