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.

The previous approach added Packages in the PackageManager directly,
completely sidestepping the scanning the PackageManager does,
which is a behavior we absolutely want to test.

By setting up the filesystem before we instantiate our `TestDub` instance,
we can remove a bunch of exceptions / overrides, and get a much closer
behavior to the final product.

This gets the unittest framework closer to the actual behavior,
and allows us to mock basic FS operations such as mkdir and writeFile.
Further improvements can be made to support more operations,
and to have package setup done before `TestDub` instantiation.