Use files instead of junctions on Windows (#11269)
Instead of using junctions, we can just write files that contain (as the
file contents) the target path. This requires a little more finesse in
that, as readers, we need to know where to expect these. But it also
means we get to avoid junctions, which have led to a variety of
confusing behaviors. Further, `replace_symlink` should now be on atomic
on Windows.
Closes #11263.
