Use `rpmmd.DepsolvePackageSets()` in Weldr API compose request handler,
instead of `rpmmd.Depsolve()`.
Extract common code from `API.allRepositories()` and
`API.allRepositoriesByImageType()` to a new method
`API.payloadRepositories()`.
Modify `API.allRepositoriesByImageType()` to return payload repositories
(repositories defined by user) as a separate slice to enable the use of
`rpmmd.DepsolvePackageSets()`, which requires the package-set-specific
repositories to be passed separately.
Keep using `rpmmd.Depsolve()` in Weldr where appropriate. The
implementation depsolves various simple package sets for multiple API
request handlers and it does not make sense to complicate the code by
moving to `rpmmd.DepsolvePackageSets()`.
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