The basic process for this is to:
1. Load the configuration using the proper loader for that version. i.e.
if you're loading V1 config, then use the V1 loader.
2. If we just loaded a version that isn't current (imagine we're
currently at V3), then we need to upgrade that config. So we first
ask the V2 loader to upgrade the V1 config to V2, then we ask the V3
loader to upgrade the V2 config to V3. We keep track of warnings and
errors throughout this process.
3. Finally, we have a current config, so we merge it into the in-process
configuration that is being loaded.
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