Declare param as Collection<Path> instead of Iterable<Path>.

Summary:
It turns out that `Path` implements `Iterable<Path>`. Therefore, to prevent
against a method accepting a single `Path` when you want a collection of
`Path`s that implements `Iterable<Path>`, we declare the type of the
constructor param to `ClasspathTraversal` `Collection<Path>` instead of
`Iterable<Path>`.

Test Plan: Sandcastle builds.