fixed: dead symlink breaks gc

Hi,

i changed the file-deletion in Composers GC function from realpath to pathname.

Why? When the garbage collection finds a dead symlink, realpath tries to resolve the symlink. Because it's impossible to resolve the symlink, realpath returns false and unlink gets called with "false" instead of a path. Satis (in this case) aborts with an ErrorException.

Changing to pathname deletes the symlink itself, not the dereferenced file. I don't know if this is a problem, but i dont think so. If the symlinks target is within the cache-root, it should be found anyways (one occurrence for the symlink itself and one for the symlinks target but i don't know for sure if the finder component works that way).

If the symlink targets is outside of the cache-dir (i don't know if that's even possible, but it shouldn't) it shouldn't handled by the garbage collection in the first way because it isn't inside the cache directory.