Add a way to specify per-project nvchecker config for upstream repos.

Individual nvchecker config files in data/upstream-nv/ must follow
naming conventions used by downstream config. When
project.upstream.nvconfig = 'foo.toml' cznicinfo uses
data/upstream-nv/foo.toml nvchecker config file to get upstream
versions.

Upstream versions are included in following commands:

* versions
* report

Refactor related code to be more universal and consistent.