Fix resolving chained symlinks with the WinFSP VFS

[tomgr]

It turns out that WinFSP's FspFileSystemFindReparsePoint only splits on backslashes when calculating reparse depth. Forward slashes in symlink targets (e.g. from VirtualSymlink) caused chained symlinks to fail. We now convert forward slashes to back slashes when returning the reparse point.

[aspect-workflows]

Test

All tests were cache hits

19 tests (100.0%) were fully cached saving 2s.

[tomgr]

Just converting to draft whilst I do a bit more testing.

[EdSchouten]

If we change this:

if err := path.Resolve(path.LocalFormat.NewParser(string(target)), &w); err != nil {

to:

targetParser := path.LocalFormat.NewParser(string(target))
if err := path.Resolve(targetParser, &w); err != nil {

Then we can also the path library to do the sanitization:

cleanPathBuilder, scopeWalker := path.EmptyPath.Join(path.VoidScopeWalker)
if err := path.Resolve(targetParser, scopeWalker); err != nil {
	return 0, err
}
cleanPath, err := cleanPathBuilder.GetWindowsString()
	return 0, err
}

I know it's more code, but at the same time it's a bit more future proof. At some point we may consider changing VirtualSymlink() and VirtualReadlink() to accept/return a path.Parser. That way the VFS API is completely oblivious of path separators. If/when that happens, this code will continue to work as expected.

[tomgr]

I've done this but it required some work on the path library to make it correctly handle more exotic windows paths: see buildbarn/bb-storage#329.

Once that's merged I'll update this PR with a reference to the appropriate commit.