refactor(build): explicitly mention src folder in imports

Export files are now directly under the module folder,
e.g. `core/core.js`. With this, an import like `core/core`
won’t need a path mapping (e.g. via `System.paths`) any more.
This adds the `src` folder to all other import statements as well.

tools/transpiler/spec/imports_spec.js

@@ -7,7 +7,7 @@ import * as fooModule from './foo';
 
 import * as exportModule from './export';
 
-import {Type} from 'facade/lang';
+import {Type} from 'facade/src/lang';
 
 import {Baz} from './reexport';