fix: VFS should not confuse paths with source roots that have the same prefix
Previously, the VFS would assign paths to the source root that had the longest string prefix match. This would break when we had source roots in subdirectories:
```
/foo
/foo/bar
```
Given a file `/foo/bar_baz.rs`, we would attribute it to the `/foo/bar` source root, which is wrong.
As a result, we would attribute paths to the wrong crate when a crate was in a subdirectory of another one. This is more common in larger monorepos, but could occur in any Rust project.
Fix this in the VFS, and add a test.
773b4a5ac7