scanner: should output the root SWHID as well

in the following example:

$ swh scanner scan -f json -x '*.git' ~/swh/git/helloworld
{
    "/home/zack/swh/git/helloworld/COPYING": {
        "known": true,
        "swhid": "swh:1:cnt:94a9ed024d3859793618152ea559a168bbcbb5e2"
    },
    "/home/zack/swh/git/helloworld/Makefile": {
        "known": true,
        "swhid": "swh:1:cnt:a8becc46666b57d34e703a90660029507dac87a2"
    },
    "/home/zack/swh/git/helloworld/README.md": {
        "known": true,
        "swhid": "swh:1:cnt:4ec6b1e9b2e32f52c13c3b4d1faa688829d2d363"
    },
    "/home/zack/swh/git/helloworld/src": {
        "known": true,
        "swhid": "swh:1:dir:b94a90cd37b572636244856a02214da21504e754"
    }
}

it would be nice to see the SWHID of the root dir as well. Ditto in the case in which the root dir is unknown.

In fact, I think there are two orthogonal issues mixed here.

1. is whether the output should stop at known dir or not (stopping should be the default, but it would be useful to have a flag to keep going when outputing stuff, even if of course there is no way to keep going when //querying the server//)
2. the fact that the output of the root dir is not shown on the output --- I think it should be always shown

I think we need to brainstorm about a clear spec for this. @DanSeraf: what do you think?

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Migrated from T2730 (view on Phabricator)