Repair manglesrcname

- correct undefined variable
- use removeprefix instead of lstrip to avoid stripping other characters

Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>

opam2rpm.py

@@ -116,8 +116,11 @@ def parse_dependencies(dependencies):
     return buildrequires
 
 def manglesrcname(name):
-    if name.startswith('ocaml-') and prefixed: # where is "prefixed" defined ?
-        return name.lstrip('ocaml-')
+    log.debug(f'Mangling srcname {name}')
+    if name.startswith('ocaml-') and not downstream_prefixed:
+        name = name.removeprefix('ocaml-')
+        log.debug(f'Rewriting srcname to {name}')
+        return name
     else:
         return name