Ignore symlinks in tree hasher

[sdboyer]

vgo has set the (omg, wonderful) precedent that we're just gonna discard symlinks from well-formed dependencies. Just, straight up, ignore them. (This is one of the things i really like about vgo).

We'll ride that train, and ignore symlinks in the tree hasher that @karrick wrote, but we only just finally started using in #1912. That'll simplify the code, as well - we can get rid of the custom walker and just use Filepath.Walk().

[karrick]

@sdboyer, I got pulled away from this project pretty much one year ago due to mounting work pressures.

That being said, I can update it to use godirwalk, while ignoring symlinks, and also be ~ 2x speed of filepath.Walk...