debian: Handle invalid version string when parsing changelog

Some packages can have an invalid version string in their changelog
which raises a ValueError when attempting to parse it in the
get_intrinsic_package_metadata method of the loader.

As a consequence each package release with such bogus entry in the
changelog was discarded from the snapshot created by the loader.

So prefer to get the raw string version instead of parsing it to
workaround that issue.

Fix #1493.

swh/loader/package/debian/loader.py

@@ -446,8 +446,8 @@ def get_intrinsic_package_metadata(
     history: List[Tuple[str, str]] = []
 
     for block in parsed_changelog:
-        assert block.package is not None
-        history.append((block.package, str(block.version)))
+        assert block.package is not None and block._raw_version is not None
+        history.append((block.package, block._raw_version))
 
     changelog = DebianPackageChangelog(
         person=uid_to_person(parsed_changelog.author),

swh/loader/package/debian/tests/test_debian.py

@@ -8,6 +8,7 @@ import datetime
 import hashlib
 import logging
 from os import path
+import textwrap
 
 import pytest
 import requests
@@ -433,6 +434,21 @@ def test_debian_get_intrinsic_package_metadata(
 
     # Retrieve information on package
     dsc_path = path.join(path.dirname(extracted_path), dsc_name)
+
+    with open(path.join(extracted_path, "debian/changelog"), "a") as changelog:
+        # Add a changelog entry with an invalid version string
+        changelog.write(
+            textwrap.dedent(
+                """
+                cicero (0.7-1_cvs) unstable; urgency=low
+
+                    * Initial release
+
+                    -- John Doe <john.doe@example.org>  Tue, 12 Sep 2006 10:20:09 +0200
+                """
+            )
+        )
+
     actual_package_info = get_intrinsic_package_metadata(
         p_info, dsc_path, extracted_path
     )
@@ -446,6 +462,7 @@ def test_debian_get_intrinsic_package_metadata(
                 ("cicero", "0.7.2-2"),
                 ("cicero", "0.7.2-1"),
                 ("cicero", "0.7-1"),
+                ("cicero", "0.7-1_cvs"),
             ],
             person={
                 "email": "sthibault@debian.org",