fetchTree cleanup

Two changes:

* The (probably unintentional) hack to handle paths as tarballs has
  been removed. This is almost certainly not what users expect and is
  inconsistent with flakeref handling everywhere else.

* The hack to support scp-style Git URLs has been moved to the Git
  fetcher, so it's now supported not just by fetchTree but by flake
  inputs.

src/libutil/url.cc

@@ -158,4 +158,21 @@ ParsedUrlScheme parseUrlScheme(std::string_view scheme)
     };
 }
 
+std::string fixGitURL(const std::string & url)
+{
+    std::regex scpRegex("([^/]*)@(.*):(.*)");
+    if (!hasPrefix(url, "/") && std::regex_match(url, scpRegex))
+        return std::regex_replace(url, scpRegex, "ssh://$1@$2/$3");
+    else {
+        if (url.find("://") == std::string::npos) {
+            return (ParsedURL {
+                .scheme = "file",
+                .authority = "",
+                .path = url
+            }).to_string();
+        } else
+            return url;
+    }
+}
+
 }