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.
auto-fix dependencies for "scheme-lib" and "srfi-lite-lib"
move "ffi/examples" from base to "racket-doc"
raco pkg: add a missing `close-input-port' on network connection
ffi/unsafe: unbreak `_list-struct`

Broken by 0b1f96ab3d (specifically one of my changes to
Tobias's patch).
add "collects/realm/README.txt"

Guide readers of _Realm of Racket_ to the new location of
the "realm" directory.
Fixed define-generics so that predicate definitions follow method definitions.

I'm not sure that #:defaults and #:fast-defaults predicates should ever refer to
methods from the same generic interface, but the behavior should be divergence
rather than an undefined variable error.
Changed set? to generic-set?.
Renamed set-immutable? to set?.
Renamed set? to generic-set? and set-immutable? to set? in tests.
fix props for moved files
scribble/sigplan: add `doi'
used wrong var name in docs

closees PR 13985
normalize-path can raise non-exn:fail:filesystem? exceptions
when this function should still be returning #f (and not
raising an exn)
move the with-stack-checkpoint wrapper code into the eval handler
so tools that override the handler can get direct access to the
unexpanded syntax objects if they want them
fix another bug introduced in efd2683301
move the teachpack/deinprogramm collection into
the deinprogramm pkg

This required making the deinprogramm pkg into a 'multi
pkg, which is why all of those other files moved
around too
fix deinprogramm props based on new location
fix deinprogramm pkg info
remove uses of scheme/* in src

Fixes windows snapshot builds.