Isolate externally-managed installation logic

[jaimergp]

This needs to be executed as part of the conda recipe, so it would be nice if the import path doesn't need conda and friends (importlib.resources is enough).

That said, I should also look into whether some kind of python packaging configuration allows to indicate that EXTERNALLY-MANAGED should be placed in the parent of site-packages. This is different for every Python version, and also across Unix/Windows. This way we don't need custom code being run at build time.

[jaimergp]

For now I just isolated the code we run to copy the file, but it would be nice(r) to have this handled natively.