github: URLs for adding remote listings

src/dds/pkg/info.cpp

 
 #include <neo/url.hpp>
 #include <neo/utility.hpp>
+#include <range/v3/distance.hpp>
 
 using namespace dds;
 
         auto     namever_path = replace(leaf, "@", "/");
         url.path = (path.parent_path() / "pkg" / namever_path / "sdist.tar.gz").generic_string();
         return http_remote_listing::from_url(url.to_string());
+    } else if (url.scheme == "github") {
+        fs::path path = url.path;
+        if (ranges::distance(path) != 2) {
+            throw_user_error<errc::invalid_remote_url>(
+                "github: URLs should have a path with two segments");
+        }
+        auto fragment = url.fragment;
+        if (!fragment) {
+            throw_user_error<errc::invalid_remote_url>(
+                "github: URLs should have a fragment naming a Git ref to pull from");
+        }
+        auto new_url = fmt::format("https://github.com/{}/archive/{}.tar.gz", url.path, *fragment);
+        return parse_remote_url(new_url);
     } else {
         throw_user_error<
             errc::invalid_remote_url>("Unknown scheme '{}' for remote package URL '{}'",