In order to properly invalidate for watchman events, we need to record that we walked symlinks in the product graph. This review adds symlink handling to `engine.exp.fs`, and improves the type safety of filesystem operations by requiring that a PathGlobs object matches exactly one of `Files`, `Dirs` or `Links`.

- Replace `fs.Path` with `fs.Stat` subclasses: `fs.{File,Dir,Link}`
- Add explicit support to PathGlobs for matching directories. A PathGlobs object matches either Files or Dirs, but not both.
- Resolve symlinks by recursively requesting `Files`/`Dirs` for a `Path` projected from a `ReadLink`
- Replace `RecursiveSubDirectories` with use of a `PathGlobs` object recursively matching `Dirs`.
- Add ProjectTree.{lstat, readlink, listdir}
- Split `test_fs.py` from `test_path_globs.py`, and prepare to test ScmProjectTree (see [#3189](#3189))
- Include tests in `test_fs.py` to validate the actual filesystem events that occurred.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/124108442

Bugs closed: 3121, 3190

Reviewed at https://rbcommons.com/s/twitter/r/3691/