nailing-cargo: Work around cargo annoyance with workspaces

nailing-cargo: Work around cargo annoyance with workspaces

When the invocation $subdir is a workspace, with -u, cargo complains
that the packages in the linkfarm are different to the ones in the
source area.

Fix this by, in this case, adjusting those path references to point
into the linkfarm.

cargo seems to use realpath on one of these paths, so we must do so
too, or cargo complains that two things are different which are
literally two different paths to the very same symlink.

With this change I am able to build lexpr-rs, both in its own right,
and as a dependency.  Closes #1.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>