Prior to this fix, cding into a symlink and then completing .. would complete
from the physical directory instead of the logical directory, which could not
actually be cd'd to. Teach cd completiond to use the logical directory.
This switches fish to a "virtual" PWD, where it no longer uses getcwd to
discover its PWD but instead synthesizes it based on normalizing cd against
the $PWD variable.
Both pwd and $PWD contain the virtual path. pwd is taught about -P to
return the physical path, and -L the logical path (which is the default).
Fixes#3350
