Split out RelPath into a separate crate (#61029)

This is necessary to remove some `util` dependencies from crates, as
well as better sharing for our projects. This also includes the WIP
AbsPath abstraction as well as some bug fixes from internal tooling.


Release Notes:

- N/A or Added/Fixed/Improved ...

crates/dev_container/src/devcontainer_api.rs

@@ -172,13 +172,13 @@ pub fn find_devcontainer_configs(workspace: &Workspace, cx: &gpui::App) -> Vec<D
 pub fn find_configs_in_snapshot(snapshot: &Snapshot) -> Vec<DevContainerConfig> {
     let mut configs = Vec::new();
 
-    let devcontainer_dir_path = RelPath::unix(".devcontainer").expect("valid path");
+    let devcontainer_dir_path = RelPath::from_unix_str(".devcontainer").expect("valid path");
 
     if let Some(devcontainer_entry) = snapshot.entry_for_path(devcontainer_dir_path) {
         if devcontainer_entry.is_dir() {
             log::debug!("find_configs_in_snapshot: Scanning .devcontainer directory");
             let devcontainer_json_path =
-                RelPath::unix(".devcontainer/devcontainer.json").expect("valid path");
+                RelPath::from_unix_str(".devcontainer/devcontainer.json").expect("valid path");
             for entry in snapshot.child_entries(devcontainer_dir_path) {
                 log::debug!(
                     "find_configs_in_snapshot: Found entry: {:?}, is_file: {}, is_dir: {}",
@@ -199,7 +199,7 @@ pub fn find_configs_in_snapshot(snapshot: &Snapshot) -> Vec<DevContainerConfig>
 
                     let config_json_path =
                         format!("{}/devcontainer.json", entry.path.as_unix_str());
-                    if let Ok(rel_config_path) = RelPath::unix(&config_json_path) {
+                    if let Ok(rel_config_path) = RelPath::from_unix_str(&config_json_path) {
                         if snapshot.entry_for_path(rel_config_path).is_some() {
                             log::debug!(
                                 "find_configs_in_snapshot: Found config in subfolder: {}",
@@ -223,7 +223,7 @@ pub fn find_configs_in_snapshot(snapshot: &Snapshot) -> Vec<DevContainerConfig>
 
     // Always include `.devcontainer.json` so the user can pick it from the UI
     // even when `.devcontainer/devcontainer.json` also exists.
-    let root_config_path = RelPath::unix(".devcontainer.json").expect("valid path");
+    let root_config_path = RelPath::from_unix_str(".devcontainer.json").expect("valid path");
     if snapshot
         .entry_for_path(root_config_path)
         .is_some_and(|entry| entry.is_file())
@@ -400,7 +400,7 @@ pub(crate) async fn apply_devcontainer_template(
             log::error!("Can't create relative path: {e}");
             DevContainerError::FilesystemError
         })?;
-        let rel_path = RelPath::unix(relative_path)
+        let rel_path = RelPath::from_unix_str(relative_path)
             .map_err(|e| {
                 log::error!("Can't create relative path: {e}");
                 DevContainerError::FilesystemError