Minor manual cleanu

* use default derive
* use `strip_prefix` where possible to avoid dup work

crates/vfs/src/file_set.rs

@@ -140,16 +140,11 @@ impl FileSetConfig {
 }
 
 /// Builder for [`FileSetConfig`].
+#[derive(Default)]
 pub struct FileSetConfigBuilder {
     roots: Vec<Vec<VfsPath>>,
 }
 
-impl Default for FileSetConfigBuilder {
-    fn default() -> Self {
-        FileSetConfigBuilder { roots: Vec::new() }
-    }
-}
-
 impl FileSetConfigBuilder {
     /// Returns the number of sets currently held.
     pub fn len(&self) -> usize {