Support generating multiple packages from packaging_tags

[MattF-NSIDC]

Description

We've discussed, for example, QGreenland for Educators as a "add-on" or "specialized" distribution of QGreenland. This PR implements config that allows tagging layers with the packages they belong in using packaging_tags config parameter.

Unfortunately, we struggled with segfaults from PyQGIS when we attempted to generate multiple project files in a single run. We don't have the budget remaining to focus on working around this, so perhaps we'll come back to it :)

Checklist

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• The PR description links to issues that it resolves with closes #{issue_number}
• Config lockfile updated (inv config.export > qgreenland/config/cfg-lock.json)
• Environment lockfile updated if needed (conda-lock)
• Version bumped if needed (bumpversion (major|minor|patch|prerelease|build))
• CHANGELOG.md updated (for user-facing changes)
• Documentation updated if needed
• New unit tests if needed