Allow separate aliasing of unclasses and classes.d

Now you can create separate aliases for unclasses and classes.d:

unfoo alpha
ununfoo beta
foo.d gamma

and the latest matching line in the file will be used:

foo ↦ foo
unfoo ↦ alpha
ununununfoo ↦ ununbeta
foo.d.d ↦ gamma.d
unfoo.d ↦ ungamma

Signed-off-by: Anders Kaseorg <andersk@mit.edu>