[FIXES #2096] fix relative symlinks of symlinks (#2454)

This makes yarn link work when $HOME/.config/yarn/link/ is itself a symlink.

----

The following commit forces symlinks to be relative:
36d73cc

This isn't ideal, although it enables some portability it breaks others.
For example, it is not uncommon for `$HOME/.config` itself to be be a
symlink: `$HOME/.config` -> `$HOME/src/stefanpenner/dotfiles`.

Now when running `yarn link` inside `$HOME/src/ember-cli/ember-cli` we
end up with:

```js
path.relative('/Users/spenner/.config/yarn/link/ember-cli',
'/Users/spenner/src/ember-cli/ember-cli');
=> '../../../src/ember-cli/ember-cli'
```

Which results in the link located at:
`/Users/spenner/.config/yarn/link/ember-cli` pointing to
`/Users/spenner/.config/src/ember-cli/ember-cli` rather then
`/Users/spenner/src/ember-cli/ember-cli`

This is because `/Users/spenner/.config` is actually a symlink pointing
to `/Users/spenner/src/stefanpenner/dotfiles/.config/`

---

This PR provides mitigate the issue raised in #2096, but reifying the
paths via `realpathSync` before deriving the relative path. This doesn't
fix all issues, e.g. symlinks cannot be changed after this reification.

src/util/fs.js

@@ -419,7 +419,7 @@ export async function symlink(src: string, dest: string): Promise<void> {
       await fsSymlink(src, dest, 'junction');
     } else {
       // use relative paths otherwise which will be retained if the directory is moved
-      const relative = path.relative(path.dirname(dest), src);
+      const relative = path.relative(fs.realpathSync(path.dirname(dest)), fs.realpathSync(src));
       await fsSymlink(relative, dest);
     }
   } catch (err) {