feat: Refactor to support generalised directory symlink traversal

[another-rex]

This PR enables full symlink directory traversal, and sets up the code to make it easier to implement ReadLinkFS in the future.

Here's the list of specific changes:

• First refactored Node into RootNode and Node.
  • This is fully contained to the first commit, so I highly recommend when reviewing to select the first commit, then select every commit after the first commit, to make it easier to see the diffs.
• Refactored Node functions to return errors.
• Replace the existing separated out symlink logic to a generalised one that is done during traversal.
  • This required moving the symlink depth variable onto the pathtree from the chainlayer.
  • Some of this might need to be refactored back out, if we need to implement Readlink, as during symlink resolving, we will need to differentiate the final file being a symlink, vs the directories on the way to the file being symlinks. Since currently we don't exactly need the functionality of the full readlink, I'll leave it as is for now.
  • I have exposed an argument to switch between resolving the final symlink, or returning the symlink as is.
• Add tests for relative symlinks, and nested relative symlinks.

[mleyvajr100]

LGTM, thanks so much for going ahead and refactoring Rex! Appreciate it a lot. Added a few comments, but minor nits + maybe adding more tests.

[mleyvajr100]

Maybe we can add another test that checks for symlink directories? Unless I missed that somewhere. Ex:

{
	name:    "open file that is symlinked via directory from filled tree",
	chainfs: populatedChainFS,
	path:    "/symlink-dir/foo,
	// "/symlink-dir" resolves to "/dir1", so we should get the virtual file with path "/dir1/foo"
	wantVirtualFile: &virtualFile{
		virtualPath: "/dir1/foo",
		isWhiteout:  false,
		mode:        filePermission,
	},
},

In the setUpChainFS function, we can define another virtual file:

"/symlink-dir": &virtualFile{
	virtualPath: "/symlink-dir",
	isWhiteout:  false,
	mode:        fs.ModeSymlink,
	targetPath:  "/dir1",
},

We could also go another step further and add multiple directory symlinks tests? We could add that on another PR given there's already lots of moving parts here.

[another-rex]

I reused the symlink dir to /dir2, and then added some more test cases. PTAL! (fyi if you didn't know, you can filter by changes since last review)

[mleyvajr100]

LGTM! Thanks for covering those cases.

[mleyvajr100]

LGTM!

[mleyvajr100]

LGTM! Thanks for covering those cases.