feat: add support for Mercurial (hg) source handling in mod.rs

- Implemented `split_hg_url` to parse Mercurial URLs and revisions.
- Added `checkout_hg` function to clone Mercurial repositories.
- Integrated Mercurial handling into the `prepare_one` function to support resuming builds.
- Added tests for `split_hg_url` to ensure correct parsing of URLs and revisions.

---

feat: introduce legacy STARBUILD conversion support in starbuild.rs

- Created `ConvertedStarbuild` and `ParsedStarbuild` structs to manage conversion data.
- Implemented `convert_starbuild_file` to parse STARBUILD files and generate package specifications.
- Added functions to handle parsing of various STARBUILD constructs, including dependencies and build phases.
- Developed a build script generator for STARBUILD, supporting custom install functions.
- Included tests to validate conversion of single and multi-output STARBUILD files.

src/builder/custom.rs

@@ -117,6 +117,19 @@ pub fn build(
             "DEPOT_PRIMARY_DESTDIR",
             install_destdir.to_string_lossy().into_owned(),
         );
+        let starbuild_workdir = if spec.sources().is_empty() {
+            src_dir.to_path_buf()
+        } else {
+            src_dir
+                .parent()
+                .map(Path::to_path_buf)
+                .unwrap_or_else(|| src_dir.to_path_buf())
+        };
+        crate::builder::set_env_var(
+            &mut env_vars,
+            "DEPOT_STARBUILD_WORKDIR",
+            starbuild_workdir.to_string_lossy().into_owned(),
+        );
         add_output_destdir_envs(spec, &install_destdir, &mut env_vars);
 
         // Ensure build script path is absolute for when we are in a sub-build-dir