lib: Collect system/platform related files

Previously, platforms was a random thing in top-level

lib/default.nix

@@ -22,8 +22,7 @@ let
 
   # constants
   licenses = import ./licenses.nix;
-  platforms = import ./platforms.nix;
-  systems = import ./systems.nix;
+  systems = import ./systems;
 
   # misc
   debug = import ./debug.nix;
@@ -42,13 +41,15 @@ in
             attrsets lists strings stringsWithDeps
             customisation maintainers meta sources
             modules options types
-            licenses platforms systems
+            licenses systems
             debug generators misc
             sandbox fetchers filesystem;
+
+    # back-compat aliases
+    platforms = systems.doubles;
   }
   # !!! don't include everything at top-level; perhaps only the most
   # commonly used functions.
   // trivial // lists // strings // stringsWithDeps // attrsets // sources
   // options // types // meta // debug // misc // modules
-  // systems
   // customisation