Tidy debusine terminology: distribution

We have several occurrences of target_distribution where "distribution" may be confusing (as in: the "Debian" distribution, or the "bookworm" dist/distribution). Alternate terms ((dist, distribution, suite, release, codename...) may also be ambiguous. Hopefully we'll use a consistent terminology across debusine.

See discussion at !738 (comment 482425)

This affects among others SbuildWorkflow and tasks.Lintian.

This ticket will be used to provide different options and decide/vote on the one we want to use consistently.

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