make `sml/replacer' symlink-aware

If I have a directory ~/long/path/to/my/project and then a shortcut symlink ~/myproject which points to it, and I want sml to replace pathname prefixes with :MYPRJ:, previously I would have to add two entries to `sml/replacer-regexp-list': one for the long pathname, and one for the symlink.

This commit makes sml smart enough to only require one entry for the long pathname, and then automatically spot that when I've loaded a project file ~/myproject/foo.py, the absolute (expanded) path matches an entry in `sml/replacer-regexp-list'.