PubDev: crash on invalid pubspec

https://sentry.softwareheritage.org/organizations/swh/issues/104694/?referrer=phabricator_plugin

ScannerError: while scanning for the next token
found character '\t' that cannot start any token
  in "<unicode string>", line 17, column 26:
      image_picker: ^0.6.7+22	
                             ^
(12 additional frame(s) were not displayed)
...
  File "yaml/composer.py", line 127, in compose_mapping_node
    while not self.check_event(MappingEndEvent):
  File "yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
  File "yaml/parser.py", line 428, in parse_block_mapping_key
    if self.check_token(KeyToken):
  File "yaml/scanner.py", line 116, in check_token
    self.fetch_more_tokens()
  File "yaml/scanner.py", line 258, in fetch_more_tokens
    raise ScannerError("while scanning for the next token", None,

while loading https://pub.dev/packages/ocg_gallery

Specifically, this line:

image_picker: ^0.6.7+22	

is followed by a tab character

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Migrated from T4583 (view on Phabricator)