More intelligent handling of .pyc and .pyo files

[takluyver]

When copying packages to the build directory, .pyc files are excluded unconditionally - unless the package is in a zip file, in which case everything is excluded. This should be refined:

• zip packages and regular directories should be handled the same way
• .pyo files should be caught as well
• It's possible to ship modules as only pyc/pyo files, to provide some basic obfuscation. So these files should only be stripped if the corresponding .py files are present.