allow re-registration of same model information

[topepo]

To avoid issues running devtools::test() and/or load_all().

The idea is to allow extension packages to (silently) overwrite the model registry environments if the same data is being registered.

If other data is trying to overwrite, we should issue and error saying

• If you are working on a different extension package, we don't allow this
• Otherwise, do a hard restart of the R session (i.e. don't load_all())

[hfrick]

material for context:

no onUnload() hooks called in load_all():
https://github.com/r-lib/pkgload/blob/e43073ca71fd9e2c3a759f472568518e8801353d/R/load.r#L62-L66

because it can cause crashes for packages using compiled code: r-lib/pkgload#148 (comment)

PR introducing this behaviour: r-lib/pkgload#133

[github-actions]

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