<check-sources> Add user-specified repository data - "expert mode"

Updated `repo_db` to provide `RepoDB` class, refactored
`oo-admin-check-sources` to use the new class instead of module
methods.

Added `--repo-config` option, so users can now specify their own
ini-formatted repository configurations

Added ability to "blend" provided repo config with built-in values,
but concealed from user for now (might not be necessary)