Refactor source spec parsing

Author: nadove-ucsc

src/azul/indexer/__init__.py

@@ -425,10 +425,13 @@ def parse(cls, spec: str) -> Self:
 
     @classmethod
     def _parse(cls, spec: str) -> tuple[str, Prefix | None]:
-        rest, sep, prefix = spec.rpartition(':')
-        assert sep != '', R('Invalid source specification', spec)
-        prefix = Prefix.parse(prefix) if prefix else None
-        return rest, prefix
+        try:
+            rest, prefix = spec.rsplit(':', 1)
+        except ValueError:
+            assert False, R('Invalid source specification', spec)
+        else:
+            parsed_prefix = Prefix.parse(prefix) if prefix else None
+            return rest, parsed_prefix
 
     @property
     def _prefix_str(self) -> str: