dependencies: avoid canonicalization for missing files

Checks whether a file exists before canonicalizing. This is a significant speed
up because canonicalizing walks the path from the root, which is much more
likely to spend time randomly stat'ing files that end up not being relevant.

This more than doubles (!) the speed of the program on my machine.

Author: tazjin

src/dependencies/cparse.rs

@@ -14,7 +14,12 @@ use tracing::{error, info, trace};
 
 /// Attempt to make the full path of head::tail
 /// returns None if that fails (e.g. path does not exist)
-fn try_resolve(head: &Path, tail: &PathBuf) -> Option<PathBuf> {
+fn try_resolve(head: &Path, tail: &Path) -> Option<PathBuf> {
+    let path = head.join(tail);
+    if !path.exists() {
+        return None;
+    }
+
     canonicalize_cached(head.join(tail)).ok()
 }