Canonicalizing the rust-project.json breaks our VFS

[Veykril]

#14430 (comment) broke rust-project.json files that emit relative paths for the root module paths. On windows at least due to nt paths being used which mismatches the entries in the VFS.

[manyinsects]

Seems to still be happening to people, even as we ourselves implemented generating absolute paths by default. See a user report here: rust-lang/rustlings#1462 (comment)