Extract a a method to encapsulate behavior and documentation. Rewrite for loop as legacy-style dictionary comprehension.

env is being modified at the same time as being iterated which leads to RuntimeError: dictionary changed size during iteration.

Dropping support for Python 3.2 and earlier Python 3 versions. Currently only log a warning, but future versions will fail.

This patch adds support for the 'python_platform_implementation'
environment marker as defined by PEP-0426:
https://www.python.org/dev/peps/pep-0426/#environment-markers

Backed out unintended changes made in f572ec9, restoring use of 'imp' module for dynamic module creation originally committed in 06ac3674 and 4c121bd24f.

Replace ValueErrors containing implicit contract about the structure of the message with a RequirementParseError explicitly joining arguments for the string representation.

Partial appears to behave differently on Python 2.6. Also, this syntax is more congruent and easier to read.

--HG--
branch : DistributionNotFound_list_requirers

Disable warning of LegacyVersion in parsed_version. This functionality should be called explicitly at whatever times are appropriate for the warning. Fixes #307.

Update EGG_NAME regular expression matcher to allow names to include dashes. Fixes failing test and fixes #307.

It is very helpful to know who required the missing package. E.g.:

    pkg_resources.DistributionNotFound: The 'colorama>=0.3.1' distribution was not found and is required by smlib.log.

Note that there was a comment: "unfortunately, zc.buildout uses a str(err) to
get the name of the distribution here..", but I did a search in the `buildout`
code and I think that is no longer true, so I think that we're free to make the
exception message more helpful without risk of breaking buildout:

    # In clone from https://github.com/buildout/buildout
    $ ag DistributionNotFound
    src/zc/buildout/buildout.py
    686:        except (ImportError, pkg_resources.DistributionNotFound):

    $ pip install --download=. --no-install zc.buildout
    ...
      Saved ./zc.buildout-2.3.1.tar.gz
    ...
    $ tar xf zc.buildout-2.3.1.tar.gz
    $ ag DistributionNotFound zc.buildout-2.3.1
    zc.buildout-2.3.1/src/zc/buildout/buildout.py
    686:        except (ImportError, pkg_resources.DistributionNotFound):

--HG--
branch : DistributionNotFound_list_requirers