@Root annotation is ignored when the 'module' output is turned on

[ghost]

To reproduce:

1. create a package org.foo.bar with a class FooBar.java
2. Add a @jsweet.lang.Root annotation on the package
3. Run the jsweet transpiler w/o the module option => observe how the generated FooBar.ts directly under the root output folder (as expected)
4. Now turn on the module output (for instance using module = 'amd' in the gradle options) => observe how this time FooBar.ts is located under a org/foo/bar sub-folder

[renaudpawlak]

This issue is actually closely linked to #64. Shall resolve both at a time.

[renaudpawlak]

Now the @Root annotation is not ignored, but you can have only one @Root annotation, as explained with mode detail at #64.