Instead of passing dictionaries around that are inconvenient to
use in code and even more in the `mpp-format-*` directives, use
a simple class to represent package information. Use that in
the `pkginfo` dict that can be accessed via `mpp-format-*`. Use
the `evra` property instead of string manipulation in the
`fedora-boot.mpp.json` and `-ostree-bootiso.mpp.json` manifest.
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