Simpler organisation to propagate the import errors in command line options.

[gabrieldemarmiesse]

In pull requests #547 and #554 we ensured that the user would get a proper import error if he/she used observers with optional dependencies.

In this pull request I make sure that the user gets a proper ImportError if using the --enforce-clean command line option.

After this, there is no need for the with_metaclass, parse_mod_deps functions and CheckDependencies class.

It was a fairly complexe piece of code and it's better if we can avoid supporting it.

[coveralls]

Coverage decreased (-0.007%) to 85.321% when pulling 79a8fb4 on gabrieldemarmiesse:remove_complicated_package_check into 6b177b3 on IDSIA:master.

[JarnoRFB]

👍