Installed type

We require an Installed type that describes the packages that have been installed to the build environment when a package was built. See repod's implementation for reference: https://gitlab.archlinux.org/archlinux/repod/-/blob/cc0e43c7c858c679027068639e7db554fb78aa21/repod/files/buildinfo.py#L134-174

Do note, that Installed is a compound type (concatenation of Name, Version and Architecture) and potentially we don't want to have it as a separate type and instead handle it in the .BUILDINFO spec directly. I'm open for suggestions here!