Fix inconsistent path slashes breaking seen logic

On windows machines, globs return paths with backslashes, but filepaths
are represented with forward slashes.  This breaks the "seen" tracking,
as d:\some\path obviously won't match d:/some/path.  Use path.resolve()
in the File's `seen` method to normalize slashes in paths before
checking against the "seen" hash, and do the same in the GlobNode's
`findFiles` method.