Watch '.lessimport' files for changes (but don't parse them).

'.lessimport' is now meaningful rather than just a suggestion in the docs.

docs/client/packages/less.html

@@ -11,7 +11,7 @@
 bundle.
 
 {{#note}}
-If you want to `@import` a file, give it another extension like `.lessimport`
+If you want to `@import` a file, give it the extension `.lessimport`
 to prevent Meteor from processing it independently.
 {{/note}}
 

packages/less/package.js

@@ -42,6 +42,10 @@ Package.register_extension(
   }
 );
 
+// Register lessimport files with the dependency watcher, without actually
+// processing them.
+Package.register_extension("lessimport", function () {});
+
 Package.on_test(function (api) {
   api.use('test-helpers');
   api.add_files(['less_tests.less', 'less_tests.js'], 'client');