Move `OSBuildMetadataToRPMs()` and `PackageMetadataToSignature()`
functions from the `rpmmd` package to `osbuild2` package to prevent
import cycles while de-duplicating `rpmStageInputs()` function from
`stage_inputs.go` of distro definitions.
Rename `PackageMetadataToSignature()` to
`RPMPackageMetadataToSignature()`, since it takes specifically
`RPMPackageMetadata` type as an argument.
Adjust affected parts of code (unit tests, cloudapi, worker).
Signed-off-by: Tomas Hozza <thozza@redhat.com>
8e6826e743