Use Absolutize for files that do not exist

Canonicalize relies on the file existing to resolve the potential
traversal, which won't work for file upload in case the file name has a
traversal inside it.

Cargo.toml

@@ -26,3 +26,4 @@ walkdir = "2"
 rayon = "1.3"
 notify = "4.0"
 tokio = "0.2"
+path-absolutize = "3.0.6"