Multi-repo on v2: files panel, @ search and previews ignore folders the .code-workspace lists #344

On v2 multi-repo (#338), the files panel, @ search, file picker and file previews only cover the project folder, on web and mobile. A folder the .code-workspace lists outside the project (../design) is invisible, and so is any listed folder that is not a Git repository. Files the workspace file does not list (team/notes.md) still show. The pre-v2 branch followed the folder list (#323, #325, #331, #333).

Repro: the team workspace from the other multi-repo issues. Open Files: it shows web, notes.md, team.code-workspace. Type @guide in the composer: nothing.

Expected: one root per listed folder, named the way the file names it (API, Design, Web App). @guide offers Design/guide.md. Previews and breadcrumbs resolve through the root that holds the file, and a chat link to an absolute path inside a listed folder opens.