Normally the repodata is generated from rpms, but we don't have those here,
so we need to maintain the repodata XMLs by hand. The repomd.xml file contains
checksums of the other XMLs and these checksums must always correspond with the
files on disk. Run update-checksums.py to update the checksums any time you make
any changes.

Records in filelists.xml and other.xml are mapped to primary.xml via the pkgid
key. Normally it's a sha256 checksum of the indexed RPM. Since the RPM file
is not available we need to generate a unique pkgid, for example:

    $ uuidgen | sha256sum | cut -d ' ' -f 1
    ec57b154a186fdc1f71976fc0fde97d51c744bc88d222828b4cfa42e3b1f855b
