swh-identify: support recursive checksumming of directories

swh-identify should support recursive checksumming of directories, in the sense of showing on output the checksums of all files (recursively) contained in the directory rather than only the checksum of the top-level directory. Associating checksums to specific files will become trivial once #1133 (closed) is completed.

Recursive listing should be optional, and enabled passing -r/--recursive.

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Migrated from T1136 (view on Phabricator)