WIP: Add test for symlinks edge case, remove check for absolute symlinks when staging artifacts

There is a chance i'm being dumb here (as always) but I cannot find a single way to trigger the codepath that checks for symlinks outside the sandbox when staging files.

This branch adds a couple of testcases for symlink handling, and removes the os.path.realpath() calls in _ensure_real_directory(). That gives a significant time saving in certain cases; on one machine it saves 20 seconds of "staging dependencies" time when running bst shell.

Partial fix for #174 (closed)