Simplify file tree implementations

[wolfs]

We should have a look what file tree implementations we have in our code base and consolidate them. See #8403 for something similar for file collections.

[adammurdoch]

There's still loads of stuff we can simplify for file collections too.

I'd like to make sure that all instantiation is done by a factory of some kind. There's really only a couple of patterns that we need to support and pushing the implementations behind a factory interface allows us to keep the number of different implementations low and make each on efficient. This structure also makes it easier to change the implementations and the services they need (for example, let's say we start using the file system mirror in the file collection implementations or we want to start firing build operations when they are queried, etc).

I'd also like to simplify how the contents of the collections are visited, when we build the graph nodes or unpack the collection to calculate the roots or when we register the watch points or query the contents. Each of these use cases has their own pattern and implementations for doing so. There's also complexity that happens because each caller needs to provide a way to resolve paths to files, whereas I think it would be simpler for each file collection to either be composed of other collections (possible lazily calculated) or can resolve its own content. Same for task dependencies too.

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