The completion feature failed to resolve relative path not start with `./`

[kunlinglio]

While typing a relative path, if it's not begin with ./ (for example, src/document/), the provided completions falls back to a candidate whose
leading / makes it look absolute, and then lists entries of the filesystem root / instead of returning no suggestions.

Although, the highlight feature can resolve this kind of path correctly. (So when finish typing this kind of token, it will be highlighted correctly)

To Reproduce

1. Open this repo as a workspace.
2. Type src/document/.

Actual behavior

Suggests home/, usr/, bin/, etc/, Users/ …, which are the contents from `/.

Expected behavior

Suggests files under src/document/, for example, language.rs, mod.rs.