fix: make topological sorting support fully cyclic dependencies (#678)

In case the graph of dependencies only consisted of a directed cycle,
`sort_topologically` returned an empty list. This was an issue when
trying to sort the dependencies of an environment containing Python 3.12
and Pip on MacOS as both depend on each other. 

Even though topological sorting theoretically requires a DAG as its
input, the existing implementation of `sort_topologically` already
handled directed cycles just not the case where all the nodes were part
of such cycles.

The fix is that if we can't find any root nodes, we start cycle
detection from a random (the first in the given list) node.

crates/rattler_conda_types/src/repo_data/topological_sort.rs

@@ -26,9 +26,20 @@ pub fn sort_topologically<T: AsRef<PackageRecord> + Clone>(packages: Vec<T>) ->
     let mut stack = Vec::new();
     let mut cycles = Vec::new();
 
-    for root in &roots {
-        if !visited.contains(root) {
-            if let Some(cycle) = find_cycles(root, &all_packages, &mut visited, &mut stack) {
+    // We must start from the roots, unless all packages are in a cycle, in which case we can start from any package
+    let starting_points_for_root_finding = if roots.is_empty() {
+        packages.first().map_or_else(Vec::new, |package| {
+            vec![package.as_ref().name.as_normalized().to_string()]
+        })
+    } else {
+        roots
+    };
+
+    for starting_point in &starting_points_for_root_finding {
+        if !visited.contains(starting_point) {
+            if let Some(cycle) =
+                find_cycles(starting_point, &all_packages, &mut visited, &mut stack)
+            {
                 cycles.push(cycle);
             }
         }
@@ -339,6 +350,7 @@ mod tests {
     #[case(get_resolved_packages_for_python(), "python", &[("libzlib", "libgcc-ng")])]
     #[case(get_resolved_packages_for_numpy(), "numpy", &[("llvm-openmp", "libzlib")])]
     #[case(get_resolved_packages_for_two_roots(), "4ti2", &[("libzlib", "libgcc-ng")])]
+    #[case(get_resolved_packages_for_rootless_graph(), "pip", &[("python", "pip")])]
     #[case(get_resolved_packages_for_python_pip(), "pip", &[("pip", "python"), ("libzlib", "libgcc-ng")])]
     fn test_topological_sort(
         #[case] packages: Vec<RepoDataRecord>,
@@ -1747,6 +1759,56 @@ mod tests {
         serde_json::from_str(repodata_json).unwrap()
     }
 
+    fn get_resolved_packages_for_rootless_graph() -> Vec<RepoDataRecord> {
+        // Pip depends on Python and Python depends on Pip since Python 3.12
+        // Below is a simplified version of their repodata.
+        let repodata_json = r#"[
+          {
+              "build": "pyhd8ed1ab_0",
+              "build_number": 0,
+              "depends": [
+                  "python >=3.7"
+              ],
+              "license": "MIT",
+              "license_family": "MIT",
+              "md5": "f586ac1e56c8638b64f9c8122a7b8a67",
+              "name": "pip",
+              "noarch": "python",
+              "sha256": "b7c1c5d8f13e8cb491c4bd1d0d1896a4cf80fc47de01059ad77509112b664a4a",
+              "size": 1398245,
+              "subdir": "noarch",
+              "timestamp": 1706960660581,
+              "version": "24.0",
+              "fn": "pip-24.0-pyhd8ed1ab_0.conda",
+              "url": "https://conda.anaconda.org/conda-forge/noarch/pip-24.0-pyhd8ed1ab_0.conda",
+              "channel": "https://conda.anaconda.org/conda-forge/"
+          },
+          {
+            "build": "h1411813_0_cpython",
+            "build_number": 0,
+            "constrains": [
+                "python_abi 3.12.* *_cp312"
+            ],
+            "depends": [
+                "pip"
+            ],
+            "license": "Python-2.0",
+            "md5": "df1448ec6cbf8eceb03d29003cf72ae6",
+            "name": "python",
+            "sha256": "3b327ffc152a245011011d1d730781577a8274fde1cf6243f073749ead8f1c2a",
+            "size": 14557341,
+            "subdir": "osx-64",
+            "timestamp": 1713208068012,
+            "version": "3.12.3",
+            "fn": "python-3.12.3-h1411813_0_cpython.conda",
+            "url": "https://conda.anaconda.org/conda-forge/osx-64/python-3.12.3-h1411813_0_cpython.conda",
+            "channel": "https://conda.anaconda.org/conda-forge/"
+        }
+      ]"#;
+
+        serde_json::from_str(repodata_json).unwrap()
+    }
+
     fn get_resolved_packages_for_python_pip() -> Vec<RepoDataRecord> {
         let pip = r#"
         [