Investigate populating the fs cache with fs watcher data

[markfinger]

Re: #88 #122

When file watchers start, they crawl the targeted parts of the file system and generate a lot of data, some of which could be fed directly into the fs cache.

The primary benefit would be reducing the overheads on fs reads for stat, modifiedTime and isFile calls. Practically, this would help to reduce the time spent on dependency resolution during an initial build.

[markfinger]

I started to implement some hooks for this, but removed them in 1b24b8d

My primary concern was the increasing complexity of the various fs abstractions, and their interactions with one another.

I still think this is worth pursuing, but not until the project (and in particular the file_system module) has stabilised.