Package variance warning on SBCL

Hello

I made a merge request (https://github.com/slime/slime/pull/776) after
commit dd179f4a changed the behaviour of loading the swank asdf system
when swank has been loaded by swank-loader already. With dd179f4a,
loading the asdf system proceeds and produces lots of redefinition
warnings and also package variance warnings on SBCL. The latter made
asdf:load-system fail when run under Slime.

I believe this fix is necessary to compile stuff that depends on swank
via ASDF. Please consider merging it.

Cheers,
G=C3=A1bor