add partial_canonicalize helper function

This is useful for checking for potential overlap between paths that
have not been fully created yet.

Cargo.toml

@@ -153,6 +153,7 @@ nix = { version = "0.29", features = ["resource", "time"] }
 assert_cmd = "2.0.16"
 insta = { version = "1.42.2", features = ["filters", "json", "redactions"] }
 predicates = "3.1.3"
+tempfile = "3.19.1"
 
 [profile.dev.package.insta]
 opt-level = 3