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/annotations_spec.js

@@ -1,6 +1,6 @@
 import {describe, it, expect, iit} from 'test_lib/test_lib';
 import {readFirstAnnotation} from './fixtures/annotations';
-import {CONST} from 'facade/lang';
+import {CONST} from 'facade/src/lang';
 
 class Inject {}
 class Bar {}

tools/transpiler/spec/for_of_spec.js

@@ -1,5 +1,5 @@
 import {describe, it, expect} from 'test_lib/test_lib';
-import {ListWrapper, MapWrapper} from 'facade/collection';
+import {ListWrapper, MapWrapper} from 'facade/src/collection';
 import {IterableList} from './fixtures/facade';
 
 export function main() {

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';