Redesign custom canonicalize, enable multiple dots expansion earlier. (#1588)

* Expand n dots early where tilde was also expanded.

* Remove normalize, not needed.
New function absolutize, doesn't follow links neither checks existence.
Renamed canonicalize_existing to canonicalize, works as expected.

* Remove normalize usages, change canonicalize.

* Treat strings as paths

crates/nu-parser/src/path.rs

@@ -0,0 +1,52 @@
+use std::path::{Component, Path, PathBuf};
+
+fn expand_ndots(path: &str) -> String {
+    let path = Path::new(path);
+    let mut expanded = PathBuf::new();
+
+    for component in path.components() {
+        match component {
+            Component::Normal(normal) => {
+                if let Some(normal) = normal.to_str() {
+                    if normal.chars().all(|c| c == '.') {
+                        for _ in 0..(normal.len() - 1) {
+                            expanded.push("..");
+                        }
+                    } else {
+                        expanded.push(normal);
+                    }
+                } else {
+                    expanded.push(normal);
+                }
+            }
+
+            c => expanded.push(c.as_os_str()),
+        }
+    }
+
+    expanded.to_string_lossy().to_string()
+}
+
+pub fn expand_path(path: &str) -> String {
+    let tilde_expansion = shellexpand::tilde(path);
+    expand_ndots(&tilde_expansion)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn expand_in_relative_path() {
+        let expected = Path::new("../..");
+        let expanded = PathBuf::from(expand_path("..."));
+        assert_eq!(expected, &expanded);
+    }
+
+    #[test]
+    fn expand_in_absolute_path() {
+        let expected = Path::new("/foo/../..");
+        let expanded = PathBuf::from(expand_path("/foo/..."));
+        assert_eq!(expected, &expanded);
+    }
+}