[FIX] migration_issue_bot: Line ending + already checked marks

[pedrobaeza]

This commit includes 2 fixes:

• Add to the regular expression that detects the line the carrier return (\r), as $ is not representing the same as it's fed from the issue text.
• For detecting if the module list has ended, we were not taking into account that there can be modules already migrated, and thus, the x should be included in such regular expression.

@Tecnativa

[pedrobaeza]

Hi, @sbidoul, it's me again, sorry. I've continued testing the command and found what I expect the last 2 bugs.

[sbidoul]

Please, please, increase test coverage on that function. It should be easy.

[sbidoul]

deployed