Simplify the usage of the file types when creating a repository

[ghost]

In my case (I don't create the sqlite files), it's much easier/nicer to use createrepo_c.XmlFile than createrepo_c.PrimaryXmlFile and friends. However:

• to create an instance of createrepo_c.XmlFile, I need a type constant (e.g. createrepo_c.XMLFILE_PRIMARY)
• I need the name of the type (e.g. primary) as well to compose the file name (e.g. path/primary.xml.gz) - (Well, I don't need it. The name can be arbitrary. But it's a common practice to use these names.)
• I need the name of the type to refer to the file in a createrepo_c.RepomdRecord instance

I can think of multiple solutions:

• there can be a mapping from type constants to their names and/or the opposite mapping
• there can be a factory function that creates file instances for given type constant (optionally with the default file name)
• there can be a "builder" that has an add_pkg method and a dump method (it will create all the needed files in a given directory)
• there can be a function that transforms a metadata file into a createrepo_c.RepomdRecord
• the createrepo_c.Repomd class can have a method that does the same as set_record but accepts metadata files
• the createrepo_c.XmlFile.__init__ can accept type names
• the createrepo_c.RepomdRecord.__init__ can accept the type constants

but I don't insist on any of them.