Provide a lists of RPMs installed by the PackageManager abstraction

[praiskup]

Mock should provide the list of RPM installation transactions that were done to prepare the build root. This would be used

• to provide a software bill of materials,
• to make the buildroot easily reproducible (the shape of buildroot heavily depends on the package installation order),
• to separate the build dependency calculator from the final buildroot preparation that can be made "isolated" (no network connectivity), see Calculate spec/srpm (dynamic) build requires as a separate build mode #1358

Some notes:

• the info provided by Mock should contain RPM locations, and hashes (e.g. in Copr, we allow re-building the same NEVRAs multiple times, so these are not unique)