Switch back to SyntaxError for invalid markers, stops consumers having to import packaging themselves

Destroy MarkerEvaluation for being pointless, fix two silly mistakes in tests and update packaging changes.

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Based on experimentation, the canonical module name needs to be in sys.modules on Python prior to 3.3, but must be omitted on Python 3.3 and later.

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Pop the module off the stack, preventing the 'Version' class from having a different manifestation in packaging than in pkg_resources.

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Create a PEP 302 importer for managing conditional import of vendored packages from the 'extern' namespace. This technique avoids the use of 'imp' and works even when setuptools is installed as a zipped egg. Ref #229.

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Move extern.packaging into a package to enable package-relative imports to resolve propertly. Ref #229.

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Use the same technique in pkg_resources, relying on an 'extern' module to resolve the conditional import.

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Modeling after Astropy's technique for bundling libraries, the imports are now much cleaner. Thanks @embray. Ref #229.

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Fix multiline strings with missing spaces

is already cleaned up by py.test.