Better handling of missing packages in DEFINE-PACKAGE :IMPORT-FROM
Closes #98 (closed) #101 (closed)
Signal a style warning if we :import-from
using a package that does not exist and an empty symbol list. Remains an error to :import-from
a package that does not exist with a non-empty symbol list.
Add some tests to try and ensure :uiop/package
and :uiop/package-local-nicknames
are not modified.