Added recursive file set resolving for the WebAppFileSetResolver

[penguinboy]

Files in subdirectories of top-level namespaces are now added as a part of that namespace.

For example, we have a namespace directory like this:

foo.soy
more/bar.soy

We used to only resolve the namespace as the single file foo.soy.

With the attached changed both foo.soy and bar.soy will be included.