fix(grouping): non-canonical ordering

The first attempt 02c44c92d0 caused
`Section.partial_cmp` and `Section.cmp` to call each other recursively.
That lead to a stack overflow when compiled without optimizations, and
an infinite loop when optimizations were enabled.

This commit now properly implements at first `Ord` for both `Section`
and `Group`, and from this derives `PartialEq` for each.

crates/pacdef_core/src/grouping/group.rs

@@ -94,10 +94,7 @@ fn is_child_of_any_dir(path: &Path, dirs: &[PathBuf]) -> bool {
 
 impl PartialOrd for Group {
     fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        match self.name.partial_cmp(&other.name) {
+        Some(self.cmp(other))
-            Some(core::cmp::Ordering::Equal) => None,
-            ord => ord,
-        }
     }
 }
 

crates/pacdef_core/src/grouping/section.rs

@@ -67,14 +67,13 @@ impl Eq for Section {
 
 impl PartialOrd for Section {
     fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        self.name.partial_cmp(&other.name)
+        Some(self.cmp(other))
     }
 }
 
 impl Ord for Section {
     fn cmp(&self, other: &Self) -> std::cmp::Ordering {
-        self.partial_cmp(other)
+        self.name.cmp(&other.name)
-            .expect("partial_cmp compares &str, which provide total order")
     }
 }