a step toward single-collection packages as default

A package's "info.rkt" file should define `collection' as a
string to name a single-collection package's collection, or as
the symbol 'multi to declare the package as multi-collection.
If `collection' is 'same-as-pkg, then the package name is used
as the collection name.

The default for `collection' is 'multi for now, but the intent
is to change the default to 'same-as-pkg after a conversion
period. Also, support for a `single-collection' definition remains
in place, but it wil be removed.

original commit: c738a6aa3eee89a82d577dd35c70eca8ed32f5b4

pkgs/html-pkgs/html-lib/info.rkt

@@ -1 +1,3 @@
 #lang setup/infotab
+
+(define collection 'multi)

pkgs/slideshow-pkgs/slideshow-exe/info.rkt

@@ -1,3 +1,5 @@
 #lang setup/infotab
 
+(define collection 'multi)
+
 (define deps '("slideshow-lib"))