setup.py symlink improvements

A couple of improvements to the `setup.py symlink` command:

* It doesn't needlessly copy the library files into the build directory and byte-compile them before symlinking. This should be a helpful speed up on slow platforms such as the raspberry pi.
* Previously, if the target in site-packages which was to be symlinked already existed, it was ignored, which would lead to silent failure if you tried to symlink over a conventionally installed version. Now it checks that the destination is a symlink, so you'll get an error if it's a regular directory, but running `setup.py symlink` again (e.g. if scripts change) will work.