QualifiedSWHID: Fix (de)serialization of 'origin' qualifier

Having the escaped URL in `swhid.origin` is inconsistent with self.path
(which is always escaped) and never what we want, because it is only
useful while serializing, which is already handled by `__str__`.

This led to swh/devel/swh-indexer#4738
where swh-deposit parsed a qualified SWHID, then used `.origin` to get
an origin URL.

Additionally, as serialization always escapes the `origin` qualifier,
this means that deserializing then re-serializing a qualified SWHID
would double-escape it.

Finally, fixing this made the test uncover that `%` was not escaped
while serializing, while `;` was, leading to incorrect (and ambiguous)
escaped URLs.

swh/model/swhids.py

@@ -322,14 +322,14 @@ class QualifiedSWHID(_BaseSWHID[ObjectType]):
             )
 
     def qualifiers(self) -> Dict[str, str]:
+        """Returns URL-escaped qualifiers of this SWHID, for use in serialization"""
         origin = self.origin
         if origin:
             unescaped_origin = origin
+            origin = origin.replace("%", "%25")
             origin = origin.replace(";", "%3B")
-            assert urllib.parse.unquote_to_bytes(
-                origin
-            ) == urllib.parse.unquote_to_bytes(
-                unescaped_origin
+            assert (
+                urllib.parse.unquote(origin) == unescaped_origin
             ), "Escaping ';' in the origin qualifier corrupted the origin URL."
 
         d: Dict[str, Optional[str]] = {
@@ -377,6 +377,9 @@ class QualifiedSWHID(_BaseSWHID[ObjectType]):
                 "Invalid qualifier(s): %(qualifiers)s",
                 params={"qualifiers": ", ".join(invalid_qualifiers)},
             )
+        if "origin" in qualifiers:
+            qualifiers["origin"] = urllib.parse.unquote(qualifiers["origin"])
+
         try:
             return QualifiedSWHID(**parts, **qualifiers)
         except ValueError as e:

swh/model/tests/test_swhids.py

@@ -490,7 +490,7 @@ QUALIFIED_SWHIDS = [
         QualifiedSWHID(
             object_type=ObjectType.CONTENT,
             object_id=_x(HASH),
-            origin="https://example.org/foo%3Bbar%25baz",
+            origin="https://example.org/foo;bar%baz",
         ),
     ),
     (
@@ -607,15 +607,15 @@ def test_QualifiedSWHID_parse_serialize_qualifiers(string, parsed):
         assert str(parsed) == string
 
 
-def test_QualifiedSWHID_serialize_origin():
+def test_QualifiedSWHID_deserialize_origin_extra_escapes():
     """Checks that semicolon in origins are escaped."""
     string = f"swh:1:cnt:{HASH};origin=https://example.org/foo%3Bbar%25baz"
     swhid = QualifiedSWHID(
         object_type=ObjectType.CONTENT,
         object_id=_x(HASH),
-        origin="https://example.org/foo;bar%25baz",
+        origin="https://example.org/foo;bar%baz",
     )
-    assert str(swhid) == string
+    assert QualifiedSWHID.from_string(string) == swhid
 
 
 def test_QualifiedSWHID_attributes():