[File System Provider] leading forward or back-slash in library id should not be treated as project root

[alexgav]

Functional impact

We show project root path completion list if you type leading forward or backslash. I can't imagine when that would be what the user wants. The library source would never be in the current project

Minimal repro steps

1. Open or add libman.json
2. Start adding a library entry using "filesystem" provider
3. Start typing in "library" field using either forward or backslash as the first character

Expected result

Not sure, maybe root of the current drive? Root of the project drive? Not the root of the project though. I can't image the user already having the library in their project

Actual result

URL/path completion for the project root

Further technical details

I think the idea of the filesystem provider is user copying files from elsewhere on their local machine or a network share to their project. I can't think of a scenario when the user would already have library in their project. FWIW Windows Explorer seems to treats \ as the root of the C drive.