[css] path completion sometimes only works for one folder level

[gregvanl]

• VSCode Version: 1.22.0 Insiders
• OS Version: Windows 10

Steps to Reproduce:

1. In the vscode-docs repo and open inproduct-releasenotes.css.
2. Try url() and trigger suggestions. You get a list of folders. Select "docs/".

There are no further suggestions to navigate the folder structure.

Found during testing #46537

[gregvanl]

[gregvanl]

path suggestions do work if you use a relative path:

[aeschli]

The problem is that 'docs' is suggested.
docs is not relative to inproduct-releasenotes.css. but to the root folder.

I think the problem is with https://github.com/Microsoft/vscode/blob/dcc822dac11fa3fe3720739bfff703e6c15ad5fc/extensions/css-language-features/server/src/pathCompletion.ts#L58
That if is not necessary, he else case should work fine for that case too.