Break out the RPZ manipulation code

[remram44]

Providing a library for reading and manipulating RPZ bundles would be a good idea. It would also remove the duplicated files.

Goes with #192

[remram44]

I plan to reorganize the PyPI packages even further for 2.0:

Design notes:

• People who just want reprozip shouldn't depend on reprounzip machinery (but code shouldn't be duplicated)
• reprozip should support multiple operating systems, so it should be possible to install reprozip but switching out the Linux ptracer component
• reprozip will contain significant OS-agnostic logic, to recognize distro packages and language packages
• It should be possible to add support for other package managers as additional PyPI packages, though I envision this will mostly be built into reprozip

[remram44]

Introduced reprozip_core with d27bd1e1