Apropos correctly sets package.  More uniform use of sly-mode for
enabling sly-trace-dialog and sly-autodoc. Make swank-loader
operational again.

* sly.el (sly-popup-buffer-mode): Implies `sly-mode'
(sly-apropos): Fix bug, pass "buffer-package" to `sly-apropos-summary'.
(sly-xref-mode): Don't disable `sly-mode'.
(sly-create-macroexpansion-buffer): No need to turn on `sly-mode'
here.
(define-sly-contrib): Fix merge blunder.

* swank-loader.lisp (init): Accept LOAD-CONTRIBS kwarg, but warn.

* contrib/sly-autodoc.el (sly-autodoc): Use autodoc only in
`sly-editing-mode and `sly-repl-mode'.

* contrib/sly-trace-dialog.el (sly-trace-dialog): Simplify
turn-on logic.
(sly-trace-dialog-shortcut-mode-map): Renamed from misleading
`sly-trace-dialog-minor-mode'.