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.

modules/benchmarks_external/e2e_test/compiler_perf.es6

@@ -2,7 +2,7 @@ var perfUtil = require('../../e2e_test_lib/e2e_test/perf_util');
 
 describe('ng1.x compiler benchmark', function () {
 
-  var URL = 'benchmarks_external/web/compiler/compiler_benchmark.html';
+  var URL = 'benchmarks_external/src/compiler/compiler_benchmark.html';
 
   afterEach(perfUtil.verifyNoBrowserErrors);