Allow mdatagen to generate packages with names other than metadata

[braydonk]

Is your feature request related to a problem? Please describe.
In contrib, receivers will soon need to make shifts in the metrics they produce to align with Semantic Conventions. Depending on what exact changes are necessary, this is really hard to do with only one metadata package, especially if you want to not break existing usage.

Describe the solution you'd like
I would like mdatagen to be able to generate packages with names other than metadata. This would allow receivers making the semconv transition to have an entirely separate yaml file and generated package with the semconv version of the produced metrics, and for example control the produced set of metrics via featuregate.

Describe alternatives you've considered
I already tried to do this all in one metadata.yaml and metadata package, but it was messy and in some cases had conflicts that were impossible to reconcile.