Avoid Nim 2.0 warning in callers of `confutils.load`.
```
Warning: 'loadImpl' is not GC-safe as it performs an indirect call via 'secondarySources' [GcUnsafe2]
```
- The config files processing was not taking into account the `name`
pragma of the configuration object fields.
- The required fields were not searched within config files before
reporting an error that they are missing.
- Fields with the same names were not supported in different case branches
- The loaded config file path can now depend on the configuration
supplied through the command-line.
