utils: Handle new edge cases in parse_external_definition
Detect malformed URLs with missing colon.
Remove any trailing slash on path as it can lead to errors with the
swh.model.from_disk.Directory
model.
Related to #3870 (closed)
Fixes SWH-LOADER-SVN-50 and SWH-LOADER-SVN-58.
Migrated from D7013 (view on Phabricator)