Clean up the merging code a bit.

Use the hashtable instead of reusing razor_importer for this.
Introduce struct razor_merger for this. All this is steps
towards making the merging policy external to the core razor_set code.

Split out hashtable functionality from importer.

First steps towards rpm installation.

Drop support for half-baked rzr file idea.

Extend rpm dumper into an rpm importer.

Add the first bits of rpm file parser.

Improve download error handling, don't use libcurl progress.

Only download yum files if they're not there.

Link to curl and download yum files automatically.

Parse filelists.xml.gz too, so we import the yum files too.