entry point raises `VersionConflict` dues to PEP440 local version identifier

Originally reported by: johnyf (Bitbucket: johnyf, GitHub: johnyf)

Two related issues:

1. If install_requires contains no local version identifier (say dd >= 0.2.1), but a development version with local version identifier is installed (say dd == 0.2.1.dev0+000), thenpython setup.py install` fails to install that dependency.

   The error message is:

    Processing dependencies for some_package==0.1.1.dev0+e155acb1bf1b393beba01bbf591ce9419e595606.dirty
    error: dd 0.2.1.dev0+3a2ae3695536f3ae6f0f2063ad925c5e085a1d10.dirty is installed but dd>=0.2.1 is required by set(['omega'])
   

   The package gets installed, so this error can be ignored, if all dependencies are already installed.
   Otherwise, this issue can be worked around with pip install --pre dd ..

2. The main problem arises with entry points. Suppose we did as described above, and installed a package with pip install --pre dd .. If the package has an entry point, then calling the entry point raises a VersionConflict about the dependency dd that was allowed to be a development version during install:

Traceback (most recent call last):
  File "~/.virtualenvs/foo/bin/some_wrapper", line 5, in <module>
    from pkg_resources import load_entry_point
  File "~/.virtualenvs/foo/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3141, in <module>
 ...
  File "~/.virtualenvs/foo/lib/python2.7/site-packages/pkg_resources/__init__.py", line 833, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (dd 0.2.1.dev0+3a2ae3695536f3ae6f0f2063ad925c5e085a1d10.dirty (~/.virtualenvs/foo/lib/python2.7/site-packages/dd-0.2.1.dev0+3a2ae3695536f3ae6f0f2063ad925c5e085a1d10.dirty-py2.7-macosx-10.4-x86_64.egg), Requirement.parse('dd>=0.2.1'), set(['omega']))

I would expect development versions to be allowed in the context, since pip was told so during installation. Not sure if this issue is pip-only, setuptools-only, or both.

Also, this error doesn't go away if I install omega with pip install --pre dd ..

• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/496

Reproduce the failure in a Python 2.7 environment using the described setup.py install and pip install --pre commands, with a package dependency containing a PEP 440 local version identifier and an entry point. Start by tracing pkg_resources entry-point loading and dependency resolution; done means the installed development dependency is accepted when the entry point is invoked without VersionConflict.