Retire `source-bundle` command

Background

As discussed in this thread the source-bundle command is "a bit of an ugly, underused, undertested beast". Since it is not too often used and its functionality is very narrow, it would make sense to fold it into source-checkout as an option and get rid of source-bundle command.

Task description

• Add bst source-checkout command - !820 (merged)
• Add --tar option to bst source-checkout
• Add an option to bst source-checkout to generate a build script in addition to the sources
• Remove source-bundle command

Acceptance Criteria

• bst source-bundle command is removed from the CLI, but its functionality is supported by bst source-checkout.

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