@export annotations cause sbcl package variance warnings

[slyrus]

If I attempt to reload cl-dbi, I get warnings of the following form:

; file: /home/sly/quicklisp/dists/quicklisp/software/cl-dbi-20191007-git/src/dbd/sqlite3.lisp
; in: DEFPACKAGE DBD.SQLITE3
;     (DEFPACKAGE DBD.SQLITE3
;       (:USE :CL :DBI.DRIVER :DBI.LOGGER :DBI.ERROR :SQLITE :ANNOT.CLASS)
;       (:SHADOWING-IMPORT-FROM :DBI.DRIVER :DISCONNECT)
;       (:IMPORT-FROM :TRIVIAL-GARBAGE :FINALIZE)
;       (:IMPORT-FROM :UIOP/FILESYSTEM :FILE-EXISTS-P))
; --> EVAL-WHEN 
; ==>
;   (SB-IMPL::%DEFPACKAGE "DBD.SQLITE3" 'NIL 'NIL 'NIL
;                         '(("DBI.DRIVER" "DISCONNECT"))
;                         '("CL" "DBI.DRIVER" "DBI.LOGGER" "DBI.ERROR" "SQLITE"
;                           "ANNOT.CLASS")
;                         '(("UIOP/FILESYSTEM" "FILE-EXISTS-P")
;                           ("TRIVIAL-GARBAGE" "FINALIZE"))
;                         'NIL 'NIL '("DBD.SQLITE3") 'NIL ...)
; 
; caught WARNING:
;   DBD.SQLITE3 also exports the following symbols:
;     (DBD.SQLITE3:SQLITE3-USE-STORE DBD.SQLITE3:<DBD-SQLITE3-QUERY>
;                                    DBD.SQLITE3:<DBD-SQLITE3-CONNECTION>
;                                    DBD.SQLITE3:<DBD-SQLITE3>)
;   See also:
;     The ANSI Standard, Macro DEFPACKAGE
;     The SBCL Manual, Variable *ON-PACKAGE-VARIANCE*

[fukamachi]

Fixed at a1a582c.