Improve signal-to-noise ratio of `brew list --unbrewed`

Currently `brew list --unbrewed` annoyingly prints out an increasing number of files under lib/python* and lib/node_modules, making it difficult to spot files that are truly unexpected.

So now these directories (and a handful of others) are ignored. I also skipped walking `etc` and `var` since they are expected to contain non-symlink files.

Potentially there are other places where Homebrew-built software installs third-party things, but I don't know what they are. More exclude patterns are welcome.