compose: Delete dead code that copies aside yum repos

In 827e711 we stopped running two yum transactions. This means the
code that detects if the repodir exists in the yum installroot will
always return false and the code is dead.
compose: Add a --add-override-pkg-repo argument

As a developer, a workflow I have for testing things is to create
an RPM, toss it into a local yum repository, then do a compose.

However at the moment to add the local overrides I have to edit the
treefile, which is annoying.  Let's add a commandline override for
this.

Note this also deletes the old "repos_data" code which was not being
used.