Merge pdbq:pkg-bug-import into pdbq:main

This implements the 'pkg-bug import schedule.json' functionality we'll be using for filing the deb-update merge bug reports.

This makes use of the recently added report_bug() to file an arbitrary number of bug reports defined in a JSON file of a format as produced by the planned-merges bash script, and incorporates some fixes/adjustments to help ensure they work together.

This can be tested locally by running `./planned-merges > /tmp/test-schedule.json; ./pkg-bug --dry-run --debug import /tmp/test-schedule.json`. However, most problems are only going to be discovered by filing real bug reports in Launchpad. To do that, I trimmed /tmp/test-schedule.json down to just this:

$ cat /tmp/test-schedule.json
{
    "bugs": [
        {
            "package": "dovecot",
            "milestone": "22.01",
            "tags": [ ],
            "status": "New",
            "title": "Merge dovecot from Debian unstable for 22.04",
            "subscribers": [ "canonical-server" ],
            "description": "Upstream: ? (Package may be lacking a debian/watch file)\nDebian: 1:2.3.16+dfsg1-3 \nUbuntu: 1:2.3.13+dfsg1-1ubuntu3\n\n\n\nDebian does new releases regularly, so it's likely there will be newer versions available before FF that we can pick up if this merge is done later in the cycle."
        }
    ]
}

The resultant bug report is here:

  https://bugs.launchpad.net/ubuntu/+source/dovecot/+bug/1945383

Next steps from here include adding 'assignee' support (maybe?), more exercising of tags, and improving the description text. Then I need to craft scripts to generate the bug data for other types of work items besides deb-updates.