Get logical imports in "wren_modules" working.

There's a lot of changes here and surely some rough edges to iron out.
Also, I need to update the docs. But I want to get closer to landing
this so I can build on it.

test/language/module/use_nearest_modules_dir/a/b/use_nearest_modules_dir.wren

@@ -0,0 +1,3 @@
+// Stops as soon as it finds a wren_modules directory, regardless of whether or
+// not it contains the desired module.
+import "foo" // expect runtime error: Could not load module 'foo'.