Assets added via a plugin are not source files

Fixes #2488.

Assets added via "no handler" already have their source files added to
`sources` via the `sources.push(relPath)` in the sourceItems loop, and
assets added via a handler should not appear as source files themselves
--- the file that triggered the handler should!

This fixes packages like `mizzao:build-fetcher` which add static
assets via a plugin.

tools/compiler.js

@@ -422,8 +422,6 @@ var compileUnibuild = function (unipkg, inputSourceArch, packageLoader,
       servePath: path.join(inputSourceArch.pkg.serveRoot, relPath),
       hash: hash
     });
-
-    sources.push(relPath);
   };
 
   _.each(sourceItems, function (source) {