move "scheme" collection to a "scheme-lib" package

Also, move remaining "srfi" libraries to "srfi-lite-lib".

In principle, "base" should depend on "scheme-lib" and
"srfi-lite-lib", and a new "base2" package would represent the new,
smaller base. But I don't think the window has yet closed on
determining the initial "base" package.

The "srfi" libraries moved to "srfi-lite-lib", instead of "srfi-lib",
to avoid creating many extra dependencies on "srfi-lib" and all of its
dependencies. The SRFIs in "srfi-lite-lib" depend only on "base",
and they are used relatively widely.

original commit: d175c3949c

racket/collects/launcher/launcher-sig.rkt

@@ -1,4 +1,4 @@
-#lang scheme/signature
+#lang racket/signature
 
 make-gracket-launcher
 make-racket-launcher

racket/collects/setup/option-sig.rkt

@@ -1,4 +1,4 @@
-(module option-sig scheme/base
+(module option-sig racket/base
   (require racket/unit)
   
   (provide setup-option^)