The filesystem abstraction needs to be known to production to avoid
too much code duplication. So far we have used functions in
`PackageManager` that the `TestPackageManager overrides.

With this change, we can remove them, and instead just pass an interface.
On the long run, we should be able to do dependency injection simply
by passing a different object (or overriding) the `Dub` class.

An initial attempt made `FSEntry` inherit from the `Filesystem` interface,
however that attempt was doomed as it conflates two different abstractions:
`FSEntry` is a "node" in the filesystem, which we want to act on in one
of multiple ways: move to another node, or take an action. On the other hand,
`Filesystem` is a more generic interface and takes a path as most of its
function's argument. A `path` and an `FSEntry` have a lot of overlap
(they are a reference to a node in a tree) and the resulting interface
would have been very clunky.