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/src/largetable/largetable_benchmark.es6

@@ -1,4 +1,4 @@
-import {getIntParameter, getStringParameter, bindAction} from 'e2e_test_lib/benchmark_util';
+import {getIntParameter, getStringParameter, bindAction} from 'e2e_test_lib/src/benchmark_util';
 
 var totalRows = getIntParameter('rows');
 var totalColumns = getIntParameter('columns');