Add support for files provided by package

[jrohel]

Very useful and missing feature. Question is how to add it to current implementation.
Example of one idea is add new table "files". Each file will get unique Id and attributes (Path, Perm, Size, ...). The package will get new Tag "File". This Tag will refer to file Id.

Given files
| Id | Tag | Value |
| f1 | Path | /bin/sh |
| | Perm | 0644 |
| f2 | Path | /bin/bash |

And repository "TestRepo" with packages
| Package | Tag | Value |
| TestA | Release | 2 |
| | File | f1 |
| | File | f2 |